Added 'hint' support, telling Mongo the proper index to use for the query.

Judicious use of hints can greatly improve query performance. When doing a query
on multiple fields (at least one of which is indexed) pass the indexed field as
a hint to the query. Hinting will not do anything if the corresponding index
does not exist.  The last hint applied to this cursor takes precedence over all
others.

Closes #203

docs/changelog.rst

@@ -5,6 +5,7 @@ Changelog
 Changes in dev
 ==============
 
+- Added hint() support, so cantell Mongo the proper index to use for the query
 - Fixed issue with inconsitent setting of _cls breaking inherited referencing
 - Added help_text and verbose_name to fields to help with some form libs
 - Updated item_frequencies to handle embedded document lookups