Revamp template code and documentation.

- Adds 3 new template functions, `browserid_button`, `browserid_js`, and
  `browserid_info`, to make embedding a login/logout button easier.
- Simplify setup documentation and make a few miscellaneous 
  documentation fixes.
- Update JavaScript with lots of fixes.

Fixes #102; Fixes #101.
Add templatetags to make new buttons work with Django templates.

Also adds in a few small improvements based on code review and updates
the changelog and authors file.
Merge branch 'master' of https://github.com/mozilla/django-browserid
Added test for browserid_extra param