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.
I believe @mattpap  is right that it will be very hard to find a "tight" bound that will work for a wide range and large number of cases on Travis. OTOH tests that take exorbitantly long or fail spuriously half the time hinder development. 

I believe for now we should choose a very small subset of simple notebooks (three?-ish) to test for basic sanity on Travis, to make sure nothing is completely borked in notebooks. They can can have a long timeout, because there are not many. We can do local testing by hand, with a "all-notebooks" flag to the test script, or with maggie's recent script.