massive documentation improvement

- format CONTRIBUTORS.txt as REStructured Text list
- change README.rst to point to the coffeestats/coffeestats-django repository
- add some docstrings to
  coffeestats.caffeine.{admin,authbackend,forms,middleware} and
  coffeestats.caffeine_api_v1.views
- fix all URLs used in documentation and templates
- add cross references and improve headline levels
- add autodoc generated code documentation in docs/code.rst
- add autodoc configuration in docs/conf.py
- include CONTRIBUTORS.txt in docs/credits
- describe deployment
- give a lot more development instructions, document directory structure
- mention XVFB, continuous integration and test coverage reports