Began implementing BaseDatabaseOperations class for every database backend. This class will be used to hold the database-specific methods that currently live at the module level in each backend. Only autoinc_sql() has been implemented so far.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@5950 bcc190cf-cafb-0310-a4f2-bffc1f526a37