Reorganized classes for attribute serialization.

Changed some depending classes accordingly and did some code cleaning as well
Documentation has been also edited to reflect changes in the way elements are displayed, since that has been affected by changes in the text serializer.
Merge branch 'text_serialization' of https://github.com/volaya/GeoGit
Remove obsolete EntityType class and improve FieldType.forBinding()
get rid of switch statement in HashObject