Adding mising six dependcy to requirements
Typo in the documentation
Update AUTHORS file.
Update requirements.txt to match requirements in setup.py.
Add tox.ini

This covers:

* python 2.6 and 2.7
* django 1.3, 1.4 and 1.5

This also updates the django versions in .travis.yml.

Fixes #200.
Add note about tox to docs
Add .tox to .gitignore.
Fix #194: Replace verify function with Verifier classes.

Add the RemoteVerifier class to replace the verify function. Instead of
just calling verify to trigger verification, you must first create an
instance of the RemoteVerifier. Verification now returns a 
VerificationResult instance.

Update the authentication backend to use the RemoteVerifier and adds a
method for subclasses to override if they want to use a custom verifier
instead.