Added support for Insert Returning

598 -> 600 examples run
535 -> 504 failures
Added Connection.execute

This causes the following test to pass
  Sequel::Model Simple Associations should handle eager loading
      limited associations for many objects
600 -> 600 examples
504 -> 487 failing
Return res.tuples (count of updates)

This brought one test forward, and fixed a buncgh of others
the yield value is still the PG::Result

600 -> 600 examples
487 -> 458 failures
Convert result values to ruby types

This uses the PG_TYPES list of converter methods to make various
result values there actual ruby types (e.g. int)

600 -> 600 examples
458 -> 273 failures
Fixed naming conflict with instance variable

I don't know where, or what was setting @columns, but it was being
set to more than the actual column names.
This might have introduced a new problem, because some of the tests
might be failing for not having all the column names they need...

600 -> 600 examples
273 -> 254 failures
Use correct schema in schema parsing

It needs to default to "CURRENT_SCHEMA", not ""CURRENT_SCHEMA"" or
"'CURRENT_SCHEMA'" or "`CURRENT_SCHEMA`". This is why one of the
tests kept complaining about a 'tags.taggings_count' column which
clearly didn't exist; it existed on a tags table in another schema.
Good thing I've been using this db for other tests.

600 -> 600 examples
254 -> 238 failures
Removed custom implementation of columns

Now that we have a correct implementation of schema parsing, this
is no longer needed, and we can just use the base implementation of
columns

600 -> 600 examples
238 -> 238 failures