First working version of automated execution of notebooks in example folder.

Very WIP... I will work more on this in the next days...

Edit: No longer a WIP.
> In any case, this is not a final solution and maybe 15 seconds is too much too.

I think it actually might be not enough. Make sure to understand that running your code locally where one or two seconds should be enough, is very different from running in an environment like Travis CI, where the same example can run for a second but also for half a minute (both giving correct results). That's why we should ultimately target for finish events and use (big enough) timeout just as a precaution measure (because examples can hang due to bugs after all).

> Do we have some candidate in the Bokeh side?

Not really. The event infrastructure is isn't well defined in my opinion, maybe besides change notification, but that's not useful in this case.