Updated AuthorizationHandler.

Logic for matching users will be added soon.
Added attribute for method-based user authorization.
Removed AuthorizationHandler from BaseManager.

The authorized user can now be retrieved using
AuthorizationHandler.AuthorizedUser.
Processed comments by teugis.
Corrected typos.
Removed some unnecessary variables.
Now allows authorization of multiple user types.
Updated AuthorizationHandler.
Added AuthorizationRequired and AuthorizationOptional attributes.

User matching and user creation implementations will follow.
Updated authorization attribute classes.
No longer expecting to receive exceptions.
Updated StringCheck (now also checks for whitespace-only strings).
Better error handling.
Merge pull request #37 from jorisvervuurt/master

Updated BaseManager, added Attribute for method-based user authorization, updated AuthorizationHandler.