Create SchemaIntrospector classes

* Refactor ConnectionAdapter table metadata methods into
  SchemaIntrospectors.

Signed-off-by: Alex Coles <alex@alexbcoles.com>
Work on parity with Active Record API

* Various fixes to ensure method signatures are the same as latest
  Active Record release.
* Begin adding proper unit specs for ConnectionAdapter.

Signed-off-by: Alex Coles <alex@alexbcoles.com>
Remove commented code from ConnectionAdapter specs

Signed-off-by: Alex Coles <alex@alexbcoles.com>