Implemented function - returns possible relations (inverse-relations as well) of a given node (or list of nodes).


Files updated:

1) models.py
   * get_possible_relations() function added to RelationType class
     - It is a static method (can directly access it using class name)
     - Returns possible relations and inverse-relations of a given node (or list of nodes) as a tuple consisting of two dictionaries, each consisting of key as ObjectId of relation-type-node and value as relation-type-node itself where:
        - Dictionary #1: Possible relations
        - Dictionary #2: Possible inverse-relations

   * GRelation class updated
     - Renamed following:
       1) relation_type_value to relation_type
       2) object_value to right_subject
     
     - Modified - relation_type: RelationType # To embed RelationType class doccument

     - Added - use_autorefs = True # To support embedding of documents

   * Triple class updated
     - Added - Built-in save() function, overridden to create version(rcs) files
     
     
NOTE: While searching from Triple, use following syntax:
            - collection.Triple.find() -- as this is different from collection.Node.find()
            - Just to avoid changed structure-format of documents.