Standardized mbed GUI

Library release of Simon Ford's GraphicsDisplay Display Library Base Class.
This is an attempt to create a single, standardized, repository of this class, to be used by all mbed graphics apps.
The goal is to expand this project into a full featured GUI library where all necessary functions are predefined.
Different forum topics (eg : see here and here) already mentioned the need for such a library.

Perhaps we can get ideas from other open source display projects like this one (also available at GitHub)