ENH: Test examples

Adds example testing to Travis. Can test locally using `make test` (which will run it) or `make examples`.
I'm not sure what you mean, but I have a feeling this line does not do what you think it does... it makes it so that a user gets a `SkipTest` message if they try to run tests but are not using `vispy` from a place we can autodetect / have access to the `examples` directory.