ENH: Test examples

Adds example testing to Travis. Can test locally using `make test` (which will run it) or `make examples`.
Something to consider: In pyqtgraph, every example begins with a line like:
```
import init_example  #  Only needed for examples  
```
This bootstraps `sys.path` to ensure that the examples use *this* version of the library and users do not need to install or use `PYTHONPATH` just to run the examples. It also turns out to be very handy for automated testing like this..