Changed SearchQuerySet to only order by search if there is any.
Removed interface of QuerySet for Django models.

This is now responsibility of the SearchQuerySet.
Simplified some tests.
Fixed some issues on the interface of SearchQuerySet.
Made minor simplifications to the code.
Added expression NotBetween.
Improved test of select without alias.
Added QuerySet.count() to count in SQL.
Improved tests cleanup by using addCleanup.
Improved __len__ and __getitem__ of SearchQuerySet.

SearchQuerySet has now a limit of 1000 results when search is used.