Implement DriverInterface class

[PeterKraus]

This PR implements the tomato.models.ModelInterface class, which is an abstract class describing the interface with which tomato interacts with it's drivers. The drivers themselves should then expose a DriverInterface class when imported.

• basic ModelInterface implementation
• refactor ModelInterface out of tomato.models, but keep the import there
• move example_counter into a tomato-example-counter module
• add a read-only driver into a tomato-psutil module

Will close #74 and close #79.