Add tests

[JohnDoneth]

Since most of the functionality abstracts over the bus by use of the DataBus trait we should be able to provide a mock struct that implements DataBus and can assert that the correct commands are being written.

[su-sd]

An enum could be used, where each variant contains a different bus type.
Than users of the API could pass the HD44780 around without the need to specify on every function that B is a DataBus.