Override toString() in model classes

* The toString method has been overridden in all the
model classes. Calling this method now returns a 
JSON representation of the object

* Removed the "name" property from <session-factory>
in hibernate.cfg.xml to prevent hibernate from throwing
a JndiException

* Eagerly fetch drop metadata (links, places, tags and media)
For this the Collections had to be changed to Sets otherwise
Hibernate throws an exception

* Identity model class and DAOs