IMP Exportar des de mongo

* Només afegint el diccionari `_inherit_fields` ja funciona exportar des de
  models mongo, pe.e. TGErrors
FIXED https://jira.mongodb.org/browse/SERVER-1752

This bug is fixed
First work on function fields for mongodb backend
Merge remote-tracking branch 'gisce/IMP_exportar_models_mongo' into gisce

Conflicts:
	orm_mongodb.py
Merge remote-tracking branch 'gisce/function_fields' into gisce
Merge remote-tracking branch 'gisce/use_count' into gisce