Added naive Users system

Added abortIfNotExists method and user resource

Added users resource on API

Added a model to prevent fake/spam users.

-Changed the user-creation method, now needs 2 steps to complete.
-Added url /createUser to activate users using a key.
-Added a method which generates random codes

Added passwords

Fixed wrong spelling

Removed white-lines and solved user-creation conflicts.

updated README.rst

PEP8 Refactor
Added Mozilla Hacks' article about Shiva
Typos in documentation
Added /about resource
MediaDir objects check existence of paths
PEP8 and some cosmetic changes
Renamed VERSION to __version__
Added contributors list to /about resource
Added subtitle to Clients
Added Shiva4J client
Client authors are now lists