UK: add django-reversion for xforms and instances, closes #555
UK: test reversion, use timezone aware datetime
Merge pull request #597 from onaio/django-reversion

add django reversion for xform and instances
PLD: add dicttoxml req
PLD: add basic auth, docs, and simpler permission check logic
PLD: organize API doc links, add submission API
PLD: add username from parmams
PLD: the order matters for the tests
PLD: get username from user if no username and not anon
PLD: add a failing json submit test
PLD: do not require the username
PLD: remove name from fixture, use ID for this
PLD: relocate json2xform, rename to dict2xform
PLD: remove dicttoxml, use existing dict2xml
PLD: handle json submissions
PLD: convert empty dicts to null in fixture
PLD: add docs for submitting a JSON XForm