Introduce backends

[youknowone]

Add backends to select user.
UsernameBackend is default backend that select users with username in
django.contrib.auth.models.User.
ProfilenameBackend is new backend that select users with field name setting in
AUTH_PROFILE_MODULE model.

[arneb]

I like the idea behind this change. But in Django 1.5 the AUTH_PROFILE_MODULE/get_profile() mechanism is deprecated and will be removed in the future:

Deprecated in Django 1.5: With the introduction of custom User models, the use of AUTH_PROFILE_MODULE to   
define a single profile model is no longer supported. See the Django 1.5 release notes for more information.

It might be a good idea to provide an additional backend which will handle the new custom user models with a custom field (e.g. nickname).

I'll have to review some other changes so please be patient but there's a great chance I'm merging this pull request soon. Thanks for your contribution!

[youknowone]

I am happy to hear you like this. I was worried a bit because this is my first code in django module.
And new django user extension model looks great. I think I will move to 1.5 soon after it released. (with new backend)

I am waiting for this request be accepted soon. I am planning one more patch to use cache to prevent db access to test new message is available.

[youknowone]

Fixed settings-related problem to install this package.

[youknowone]

any progress or requirements to be merged?