Allow the auth. in domain config to be a Class

My env: Python 3.4.1 @ Windows 7 x64

Hi,

When I tried to follow http://python-eve.org/config.html#domain-configuration to set a Class as the auth of a domain, it failed.

So I referred to [flaskapp.py:144](/nicolaiarocci/eve/blob/develop/eve/flaskapp.py#L144) and tried this fix.  It seems to work for me.

I'm very new to Python so I don't know if this is proper or not.  And I don't know how to write test or doc either.  So I just want to contribute my idea in case you will like it.