sqlalchemy/dialects/postgresql/__init__.py:
- Added import references to JSON class

 sqlalchemy/dialects/postgresql/base.py:
 - Added visitor method for JSON class

 sqlalchemy/dialects/postgresql/pgjson (new):
 - JSON class, supports automatic serialization and deserialization of json data, as well as basic json operators.
Merge branch 'rel_0_9' of https://github.com/nathan-rice/sqlalchemy into rel_0_9