Enhanced multiple entities and their mappings

 - Added a `WordpressContentInterface`
   Comment and Post entities share a common `getContent` method. Made an
   interface for that to better react on these types.
 - Added the fluent interface for all entities
   See: http://en.wikipedia.org/wiki/Fluent_interface
 - Fixed some entity mappings
 - Fixed the service definition for `CommentManager`