Replaced monkey patching with explicit Backend

Monkey Patching often doesn't work (it didn't for me).

Reason: if the patching of auth.authenticate is done after auth.authenticate
is already imported elsewhere (e.g. in django/contrib/auth/forms.py) it is
never updated in that location.

This is especially dangerous for 2 reasons:
1. Import order may change for any reason (not really controlled by us)
2. If it changes there is no error condition, but BruteBuster will be silently
        turned off.
Added INSTALL from google-code page