* add/cleanup tests

* tidy test code and fix my typo 

* table_exists? scoped by schema search path unless schema is explicitly named. Added tests and doc to clarify the behaviour
* extract_schema_and_table tests and implementation extended to cover all cases
* primary_key does not ignore schema information
* add current_schema and schema_exists? methods
* more robust table referencing in insert_sql and sql_for_insert methods

Fix a couple of helper requires that were missed. This was causing the tests to fail to run on 1.8.7.

Refactor Active Record test connection setup. Please see the RUNNING_UNIT_TESTS file for details, but essentially you can now configure things in test/config.yml. You can also run tests directly via the command line, e.g. ruby path/to/test.rb (no rake needed, uses default db connection from test/config.yml). This will help us fix the CI by enabling us to isolate the different Rails versions to different databases.

[Fixes #439]

Applied Patch from https://rails.lighthouseapp.com/projects/8994/tickets/5577-suport-from-tsvector-data-type-in-postgresql

- Added IdentityMap to be included into AR::Base
- Fixed bug with Mysql namespace missing when running tests only for sqlite
- Added sqlite as default connection