Fixed #39 -- Added support for custom user model (Django 1.5+)

The patch is very short and most of the work was getting the tests to work
nicely in all supported Django versions. This patch almost doubles the size
of the full test suite (Travis / tox).

Thanks to Edward D'Souza for the report and initial patch.
Changed account alias for token generator

Previously, it was `username@sitename`; however as the username can also
contain an e-mail address, the identifier would contain two at signs. To
overcome this, it has been changed to `sitename (username)`. Thanks to
Edward D'Souza for reviewing the custom user model support.
Renamed tox's custom user model test suites
Added a list of all contributors

To add your name, submit a pull request or ping me.
Next release will be 0.6.0
Pulled new/updated translations
Cleaned up release notes
Test against Django 1.7b3
Updated docs
Added management commands

* status command for getting two-factor status for the given users
* disable command for disabling two-factor for the given users
WIP
Setup and login working with YubiKey (WIP)
Fixed issue with get_device on yubikey
Fixed failing tests, should also work without YubiKey
Fixed some more failing tests
Include YubiKey in the README
Improved installation instructions
Fixed error message for missing validation service
Created unit test for setup wizard
Extracted tests for YubiKey