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
Fixed #36 --Support for YubiKeys

YubiKeys offer a OTP, similar to token generator application. The key is
generated by a hardware device, generating a 44 character OTP, which is to be
by YubiCloud (or locally if needed be).
Updated Travis CI / tox build matrix

Included tests for YubiKey support.

On Travis CI a simplified build matrix is used, covering all Python versions
and all Django versions, but only for the most common combinations of the two.

On tox, a full cross-check with all supported versions and options is
performed. Hint: use detox to run the tests in parallel.
Combine test coverage for Python 2/3
Fixed #44 -- Don't require re-login after setup

Thanks to Dmitry Molotkov for the report.
Small code readability improvements
Fixed #50 -- YubiKey is requested when entering backup token
Also run flake8 on Travis CI
Django 1.7 and Python 3.4 are supported
Fixed #49 -- Advise to add backup devices after setup