Add a small test suite + continuous integration on travis

The test suite is minimal, we just check if the  project compiles fine and if we are able to import pygs.QxtGlobalShortcut.

PyQt5 is not tested on travis because travis is still running Ubuntu 12.04 and does not have any native packages for PyQt5.

(squashed commit)
Merge pull request #6 from ColinDuquesnoy/test_suite

Add a small test suite + continuous integration on travis