Take the schema into account when looking up Pg unique constraints.

This fixes the "ERROR: An object of class
UR::DataSource::RDBMS::UniqueConstraintColumn already exists..." error
due to us having multiple tables (in different schemata) with the same
name and primary key.

Change-Id: I67816888ecee58dc11ee942d2ee4fcde6064ea87
Update other RDBMS types to take schema into account.

Change-Id: Idf648c6f023c706f832077f327394d1d512f0122
Test for sqlite unique index lookups across schemata.

Change-Id: Iba70ba6834a94937daa2ff062ddd5f132bcfdf2c
Merge pull request #16 from tmooney/master

Take the schema into account when looking up Pg unique constraints.