Rename 'Rating' to 'Vote', update settings.py

* Models: Rename Rating to Vote
* Update settings.py to Django 1.7 version
* Minor changes
Models adjustments

* Use PositiveInteger fields instead of Integer fields where it fits
* Fix Comment to refer to a submission, not a file
* Reduce maximum length of a submission's name and filename to 128
* Remove tags CharField. To be replaced.
settings.py: Convert spaces to tabs
Models: Remove PlatformUser

The only addition of PlatformUser to User is loginFailureCount. For reimplementing this, we should instead use existing apps, for instance:
https://github.com/brianjaystanley/django-lockout
Add django-allauth app for account management

* Add requirements.txt
* Update README (-> README.md)