- add version check for at least 06, tests for 07 in selected

- add version check for at least 06, tests for 07 in selected
areas
- add "requires 07" decorators to test suite
- add tests for PG ENUM in offline mode.  works in conjunction
with the latest 0.7.4 tip of SQLAlchemy, fixes #9.  Docs will
be needed to illustrate how ENUM should be used.
- add support for table before_create and after_create events
within op.create_table().   Currently this will do the ENUM
thing for PG but will also invoke any other kinds of events
that might get configured on the table.