ENH: Test examples

Adds example testing to Travis. Can test locally using `make test` (which will run it) or `make examples`.
* I guess this requires the user to press `esc` for each window? It would be nice to automatically terminate the process after ~1sec so the tests can run unattended as well.
* What happens if an exception is caught inside an event loop? 

I had to do a lot of code twisting to get the subprocess handling to work across all platforms; you might find some of this useful: https://github.com/pyqtgraph/pyqtgraph/blob/develop/examples/__main__.py#L228