add basic client tests
This is a small set of very basic unit tests for the DeviceProxy. (Actually it should probably be called that instead of "client".) It requires the TangoTest device to be installed. There are some questionmarks around how the running of the server should be handled, particularly about port handling. Also there are a number of limitations when running without a DB.
Anyway, it's a start. View this as a WIP, not yet ready for merging.