Add "type" for title, summary and rights in Entry

Hi 

as defined in the RFC, the elements title, summary and rights may have a type attribute ("text", "html", "xhtml"); I have reused the implementation of Content to provide a new Text class and reworked the attributes in the Entry class. 

To keep compatibility I have kept the setters and getters to work with the simple strings, and added a new set of accessors for the complex element.

Regards
Joe