Loosen dependencies a bit

Work on support for Django 1.7
Loose dependency on http_signature a bit
Update setup.py
Require 0.2.0 development version
Simplify regular expression and actually make it work.
Merge branch 'master' of git://github.com/etoccalino/django-rest-framework-httpsignature
Merge pull request #5 from EnTeQuAk/master

Loosen dependencies, fix signature parsing regular expression