- more for #157:
- the ``op.create_table()`` directive will be auto-generated with
  the ``UniqueConstraint`` objects inline, but will not double them
  up with a separate ``create_unique_constraint()`` call, which may
  have been occurring.  Indexes still get rendered as distinct
  ``op.create_index()`` calls even when the corresponding table was
  created in the same script.
- the inline ``UniqueConstraint`` within ``op.create_table()`` includes
  all the options like ``deferrable``, ``initially``, etc.  Previously
  these weren't rendering.
- fixed the index tests to make sure the connection closes after each test
- _render_unique_constraint() and _add_unique_constraint() both call into
a common function now
- call _alembic_autogenerate_prefix within the add_index/drop_index renders