This PS implements the sort filter, allowing (for now)
the GET /revisions and GET /revision/{revision_id}/documents
endpoints to be sorted as per the API documentation in
Deckhand [0].
An additional filter has also been added to the 2 aforementioned
endpoints as well -- order -- which determines the order in
which sorted results are returned: "asc" for ascending
order and "desc" for descending order.
[0] http://deckhand.readthedocs.io/en/latest/api_ref.html#get-revisions-revision-id-documents
Change-Id: Ifb9e15b8379b0a28889a14c331d81d9a4147f1d4
16c7ec196f