Use signal and command to provide the same capability of the custom database backend.

Current custom database backend does 3 things:

 1. Call register_hstore for each connection.
 2. Install hstore contrib for every database created.
 3. Create index for hstore field.

This commit use connection_created to do the first two things. And sqlhstoreindexes command to do the third thing.

Plus this commit also fixed an issue when using south to add an hstore field with a default value.
Fall back to JsonField when using sqlite database.
Adding postgis+hstore DB-backend
Adding support of spatial queries
Switch to using django's get_version to deal with isntalling hstore
fixing geodjango bug #16778 by adding adapter
fixing setup.py
finally fixed 0x00-bug
Fix the bug that one has to install psycopg2 to use the sqlite database
Updated list of authors and year in license file.
Bumped to 1.1.2.
gt, gte, lt, lte lookup
Work in progress Json dict support.
Add json key-value support. Now we could store numbers, dicts and arrays in HStoreField.
Bugfix for processing normal keys.
Fixed the south migration bug.
Fix the print HStoreDict bug.
__contains operation supports several values for one key
Merging changes from django codebase
Adding support for Django 1.5: no more connection._version