Fixed #7420 -- Abstracted some more database options into DatabaseFeatures --...

Fixed #7420 -- Abstracted some more database options into DatabaseFeatures -- supports_usecs, time_field_needs_date, interprets_empty_strings_as_nulls and date_field_supports_time_value -- and changed various hard-coded 'if DATABASE_BACKEND == oracle' statements to use the new options. Thanks to ramiro for the patch

git-svn-id: http://code.djangoproject.com/svn/django/trunk@7643 bcc190cf-cafb-0310-a4f2-bffc1f526a37