refactor omniauth configuration method

This refactoring allows a user to use omniauth providers which do not
use ```app_id``` and ```app_secret``` without needing to change the
devise configuration.
Added support for Atlassian Crowd

Added images taken from Atlassian's website which have been cropped and resized
http://www.atlassian.com/dms/wac/images/press/Atlassian-logos/logoCrowdPNG.png