Fix combining LoginRequired with other access mixins

[mikebryant]

Make it possible to combine LoginRequiredMixin with other AccessMixins. This allows views which will redirect to login if unauthenticated, but raise an error if the user is unauthenticated but isn't allowed.
This stops the redirect loop in #161.

I'm using the approach in #200, which avoids api changes.

The tests pass (at least the versions I have available to test):

  py27-django15: commands succeeded
  py27-django16: commands succeeded
  py27-django17: commands succeeded
  py27-django18: commands succeeded
ERROR:   py32-django15: InterpreterNotFound: python3.2
ERROR:   py32-django16: InterpreterNotFound: python3.2
ERROR:   py32-django17: InterpreterNotFound: python3.2
ERROR:   py32-django18: InterpreterNotFound: python3.2
ERROR:   py33-django15: InterpreterNotFound: python3.3
ERROR:   py33-django16: InterpreterNotFound: python3.3
ERROR:   py33-django17: InterpreterNotFound: python3.3
ERROR:   py33-django18: InterpreterNotFound: python3.3
  py34-django15: commands succeeded
  py34-django16: commands succeeded
  py34-django17: commands succeeded
  py34-django18: commands succeeded

I've verified the tests fail, when run against the previous version of _access.py

The changes comply with pep8

Fixes #161.
Fixes #181.
Closes #196.
Closes #200.

[chrisjones-brack3t]

I'll look over the PR more this evening but a quick scan of it looks good.

[wildd]

Is this going to be merged in the main branch soon? I have the same problems with many login redirects when user is already logged in, for example while using "GroupRequiredMixin". Thanks!