Add support for MongoDB (or any other database) for property read-write

[VigneshVSV]

There are certain settings in hardware which are helpful to be backed up to databases as the hardware "forgets" the setting when power cycled. It may also go to a default setting.

Since settings are generally modelled as properties, after the property write, the value can be stored to the database.

Currently SQLite and MySQL/postgres is supported.

It's useful to have a JSON driven DB as the default seriliazer is JSON for all operations. It also helps the package end-developer to stick to JSON data types.

Involved tasks -

• subclass ThingDB for mongo
• implement all operations which are imported for SQLite or other databases
• support auto-setup in case use_default_db is set to True at Thing.init
• verify working at Property._post_value_set
• write tests or add it to the tests in test_property.py

If interested, do the same for another database binding (please copy this issue and propose the binding).