- Updated documentation for JSON class

 - Added support for additional operators
 - Made return as json default (rather than text)

 - Removed unneeded import of psycopg2.extensions

 - Fixed reference to HSTORE
 - Corrected spelling of SQLAlchemy

 sqlalchemy/dialects/postgresql/psycopg2:
 - Added psycopg2 specific wrapper type for JSON which uses inherent json deserialization facilities
 - Added code to detect and utilize the JSON wrapper if psycopg2 >= 2.5

test/dialect/postgresql/test_types:
- removed reference to use_native_hstore

- 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.

have been modified, such that the COLLATE operator is now of lower
precedence than the comparison operators.  This has the effect that
a COLLATE applied to a comparison will not render parenthesis
around the comparison, which is not parsed by backends such as
MSSQL.  The change is backwards incompatible for those setups that
were working around the issue by applying :meth:`.Operators.collate`
to an individual element of the comparison expression,
rather than the comparison expression as a whole. [ticket:2879]

Fix MSSQL dialects visit_drop_index to use the correct DDL

the primaryjoin examples there