Corrected response type of `PPMS.revision` command; Fixes #155
Renamed `slave.core.InstrumentBase` to `slave.core.Driver`.

It is much shorter and more meaningful.
Renamed `slave.core.py` to `slave.driver.py`.

This changes the module name to a more meaningful name, since it's not
the core of the library but implements another abstraction layer.
Merge branch 'feature-driver' into develop