- backport unique constraints reflection to 0.8.4, thereby

- backport unique constraints reflection to 0.8.4, thereby
assisting with alembic installations that have upgraded and are dealing with
PG index/unique constraint reflection.

Inspection API already supports reflection of table
indexes information and those also include unique
constraints (at least for PostgreSQL and MySQL).
But it could be actually useful to distinguish between
indexes and plain unique constraints (though both are
implemented in the same way internally in RDBMS).

This change adds a new method to Inspection API - get_unique_constraints()
and implements it for SQLite, PostgreSQL and MySQL dialects.