`migration_keys` includes `name` but `name` is not a column option.

The current code base is not uniform. After some discussion,
we have chosen to go with double quotes by default.

Comments are specified in migrations, stored in database itself (in its schema),
and dumped into db/schema.rb file.

This allows to generate good documentation and explain columns and tables' purpose
to everyone from new developers to database administrators.

For PostgreSQL and MySQL only. SQLite does not support comments at the moment.

See docs for PostgreSQL: http://www.postgresql.org/docs/current/static/sql-comment.html
See docs for MySQL: http://dev.mysql.com/doc/refman/5.7/en/create-table.html

Follow up to #18228.

In MySQL and PostgreSQL, primary key is to be `NOT NULL` implicitly.
But in SQLite it must be specified `NOT NULL` explicitly.

Before:

```ruby
create_table "big_numbers", force: :cascade do |t|
  t.integer "bigint_column", limit: 8
end
```

After:

```ruby
create_table "big_numbers", force: :cascade do |t|
  t.bigint "bigint_column"
end
```

Actually `:name` and `:type` are not column options.

A return value of `schema_type` is used by:

1. primary key type: using as `symbol.inspect`
2. normal column type: using as `symbol.to_s`

It is better to return symbol.

Example:

    create_table :posts do |t|
      t.datetime :published_at, default: -> { 'NOW()' }
    end

Current master branch includes many schema dumping improvements.
It extract these features to the appropriate files.

When working on engines that supports multiple databases, it's
very annoying to have a different schema.rb output based on which
database you use. MySQL being the primary offender.

This patch should reduce the disparities a bit.

Some databases like MySQL allow defining collation charset for specific
columns.

`precision: 0` was not dumped by f1a0fa9e.
However, `precision: 0` is valid value for PostgreSQL timestamps.

The various databases don't actually need significantly different
handling for this behavior, and they can achieve it without knowing
about the type of the object.

The old implementation was returning a string, which will cause problems
such as breaking TZ aware attributes, and making it impossible for the
adapters to supply their logic for time objects.

Remaining are `limit`, `precision`, `scale`, and `type` (the symbol
version). These will remain on the column, since they mirror the options
to the `column` method in the schema definition DSL

If it is not a default primary key, correctly dump the type and options.

Allows :limit defaults to be changed without pulling the rug out from
under old migrations that omitted :limit because it matched the default
at the time.

If we want to have type decorators mess with the attribute, but not the
column, we need to stop type casting on the column. Where possible, we
changed the tests to test the value of `column_defaults`, which is
public API. `Column#default` is not.

We're not longer using `ipaddr` in schema dumper

This removes the case statement in `SchemaDumper` and gives every `Type`
the possibility to control the SchemaDumper default value output.

/cc @sgrif

ActiveRecord::ConnectionAdapters::Column

See
https://github.com/rails/rails/blob/28b8ca766e3e7c6c43d3ae900c99f8377153c62/activerecord/lib/active_record/connection_adapters/column.rb#L16

Having column related schema dumper code in the AbstractAdapter. The
code remains the same, but by placing it in the AbstractAdapter, we can
then overwrite it with Adapter specific methods that will help with
Adapter specific data types.

The goal of moving this code here is to create a new migration key for
PostgreSQL's array type. Since any datatype can be an array, the goal is
to have ':array => true' as a migration option, turning the datatype
into an array. I've implemented this in postgres_ext, the syntax is
shown here: https://github.com/dockyard/postgres_ext#arrays

Adds array migration support

Adds array_test.rb outlining the test cases for array data type
Adds pg_array_parser to Gemfile for testing
Adds pg_array_parser to postgresql_adapter (unused in this commit)

Adds schema dump support for arrays

Adds postgres array type casting support

Updates changelog, adds note for inet and cidr support, which I forgot to add before

Removing debugger, Adds pg_array_parser to JRuby platform

Removes pg_array_parser requirement, creates ArrayParser module used by
PostgreSQLAdapter