SASL Refacturing

This pull request clean up the sasl authentication. `Sasl.js` was refactured and all auth-methods were separated into separate files. Additionally the server side component is now able to properly implement more authentication methods than PLAIN. As an example I implemented XOAuth2. The refacturing should be backward compatible but is only tested for PLAIN and X-OAUTH2. Further tests for X-FACEBOOK-PLATFORM, DIGEST-MD5, EXTERNAL should be added at a later stage.