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 without any error. Everything is explained in the [readme](https://github.com/ColinDuquesnoy/pygs/tree/test_suite#testing) :wink: 

PyQt5 is not tested on travis because travis is still running Ubuntu 12.04 which does not have any native packages for PyQt5.

To enable CI integration you must register on Travis CI and enable your repo. Tests will run the first time you push so I suggest you to register before merging the PR. Here is how it looks for in my fork: https://travis-ci.org/ColinDuquesnoy/pygs

I also changed ``QT_API`` to ``QT_SELECT`` so that we can specify both the pyqt api and the qmake executable using only one environment variable (#5) . I adapted the readme consequently.
Perhaps we could add a print to stderr to notify the user that the test did not run because PyQtX is not available on their system