Add Regexp and NotRegexp nodes for PostgreSQL

This reverts commit 6d3ed6d9, reversing
changes made to a35fede6.

small changes visit_Arel_Node_And

Added right and full outer joins

* rm_a:
  take in to account quoted and translated values

Removed all the fiels in lib/arel/visitors/ which needs dependency on 'a' also fixed the test case for : test/visitors/test_to_sql.rb:22 which pass in the parameter attribute e.g the parameter a.

If we add the casting node to the ast at build time, then we can avoid
doing the lookup at visit time.

Assign value from attribute test

fixed the readme per discussion w/ tenderlove

 The evaluates the assignment of two unqualified columns.

Such queries become possible for mortals:

```ruby
table.where(table[:qux].eq 'zomg').compile_update(table[:foo] => table[:bar])
```

Arguably much more sane than using `engine.connection.quote_column_name`
or `engine.connection.visitor.accept` on the right hand (bar), which is
totally leaking the abstraction.

Removed deprecated `Arel::Sql::Engine` and make use of `FakeRecord::Base` inplace directly for test helper.

Add tests for Nodes::Equality and Nodes::NotEqual to the ToSql class.

Extract comparison array to a constant for the time being until removing the checks completely.

Bump minitest to 5.1.

PostgreSQL bugfix for invalid SQL in subqueries

Add tests for PostgreSLQ bugfix regarding invalid SQL in subqueries when using matches() or does_not_match().