expose SchemaVisitor in the compatibility layer

sqlalchemy-migrate uses SchemaVisitor. It was one of the only
objects that changed namespaces without compatibility that we use.

Would love to get this added to remove a work around we need at
https://review.openstack.org/#/c/66156/
Merge pull request #57 from sdague/master

expose SchemaVisitor in the compatibility layer
- pullreq github 55 is in the wrong changelog!  0.9.2...
- changelog for pullreq github 57