Update to latest version of draft 3

Thanks for the PR, and sorry for the delay :(
I had to work on it a bit to get it to work, since the draft (and httpsig) changed a little:

  * Use (request-target) instead of (request-line)
  * Fix the tests
  * Add utils/sign3.py script to simplify adapting tests.

Please, accept this pull request, and update [your own](https://github.com/etoccalino/django-rest-framework-httpsignature/pull/4), and I'll gladly merge it in :+1: