Adds 'upload fixtures through API' functionality

Adds following functionality http://manage.dimagi.com/default.asp?61884#357275. Implements a view to upload fixtures.

I haven't yet added proper URL structure that agree with current API. I am looking forward to change this after discussing with you. Currently following is the URL structure to upload fixtures.
> curl -F "file-to-upload=@sample_fixture.xlsx" -v --digest -u sreddy@dimagi.com:password http://127.0.0.1:8000/a/{domain}/fixtures/fixapi/ 

It responds with a json output of the format {"code": (depending on success/failure/warning), "message": (helpful message to debug)}
Currently the codes are randomly chosen just to exemplify the implementation.
 
Also merged another [feature to download & re-upload fixture data as excel](https://github.com/dimagi/commcare-hq/pull/454)