Removed django_ldap_auth

This should make the code easier to maintain as it no longer
depends on an entire library. A signal has been implemented in
studerende/__init__.py. This should be moved once django 1.7
is released.
See http://stackoverflow.com/questions/2719038/where-should-signal-handlers-live-in-a-django-project
Updated README.md to reflect last commit

Updated todo in readme.