TSUNE-105 Split requirements into sub-configurations
TSUNE-105 Add requirements.txt for Heroku deployment
TSUNE-105 Split settings.py into smaller chunks

- Remove a lot of unused settings
- Create some kind of inheritance structure
- Refactor settings code

Detailed explanation of the new files:
- environment.py: helper code for accessing environment variables
- base.py: main template, used by all configurations
- ci.py: configuration for continuous integration server (jenkins)
- heroku.py: base template for heroku deployment
- production.py: configuration for running on heroku
- testing.py: configuration for testing on heroku
- vagrant.py: configuration for vagrant vm

Don't forget to set your OAuth secrets in PyCharm!
TSUNE-105 Fix small problem related to DATABASE_URL
Remove duplicate line
Import the correct settings module