example: build widget wrapped with PyQt

This branch demonstrates how to wrap a simple Qt widget using PyQt5 (and Python 3).
The script `test.py` in the `pyqt` directory will show the wrapped widget from Python.

Travis runs in this branch; it downloads and compiles SIP and PyQt5 at the moment.

TODO before considering merge:
  
  - [ ] make sure that Knossos also compiles if PyQt5 is not found.
  - [ ] actually wrap some useful knossos widget