Add descending field options to order_by when using order_by=True

Referencing #91:

When using order_by=True, "-fieldname" parameters are not recognized. This fix adds the descending values to the order_by choicefield. Added regression tests for the currently working case (passing field names to order_by) and the currently broken case (order_by=True).

Modified the existing ordering form tests to reflect the fact that the descending sort orders should be available. 

This was the most pythonic way I could think of accomplishing this. I don't have too much experience with django-filters so let me know if i'm completely off :P 


Fixes #91