Add Exception removed in flask.

``JSONHTTPException`` class (and in fact the entire module) was removed from Flask in this commit:

https://github.com/mitsuhiko/flask/commit/4c27f7a8c4bbe6621681178be716e6270067a3ad

There are some options that flask-restless has:

  1. Reimplement the ``JSONHTTPException`` class so that it can be used in ``json_abort``.
  2. Change ``json_abort`` to not require the class.
  3. Change places that use ``json_abort(404)`` to just use ``abort(404)``.
  4. Tie restless with version 0.9 of Flask.

Since the code base suggests we're only using ``json_abort`` for 404 messages I think that *probably* the correct solution is 3. However, because I have no idea what other people are using restless for I have implemented 1 here for a more backwards-compatible, don't-rock-the-boat solution.