ENH: Test examples

Adds example testing to Travis. Can test locally using `make test` (which will run it) or `make examples`.
No, the user does not have to press escape. The examples just have to be
formatted properly, which they are now. No event loops are entered either.
This runs on travis and locally just fine.

It should be easy enough for us to enforce a couple restrictions on new
contributions regarding what to call canvases and how to start event loops
to keeps things simple (and as a nice side effect, more uniform).