Implementing KbModels.

The use of these models, will allow to add a level of abstraction to the plugins.
Now, the source plugins are expected to produce KbModels, and the destination
plugins to read KbModels.
Implementing the KbModels in the pydesk plugin.
Implementing the KbModels in the OSD plugin.
Implementing the KbModels in the OnDisk plugin.
Implementing the KbModels in the Database plugin.
Merge pull request #7 from rgreinho/use_universal_type

Use universal type