Schema fixes on rendering of alembic op. directives

Added schema parameter in _drop_index, _drop_constraint
Removed prepending of the schema name in _add_index table
parameter.
Autogenerate index and unique's comparison takes schema into account

The inspector now will recover the indexes and unique constraints of
the table in the correct schema. Previously it was not taking schema
into account, resulting in always re-creating the existing uniques
and indexes.
Merge https://bitbucket.org/dtheodor/alembic/branch/master into m
- fix up the "build" paths
- changelog for pullreq 17
- unit tests for schema-level unique/index detection
- cleanup a bit for the get indexes/uniques if only table present