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 fact, I had to increase to 20 seconds in my local machine to see all the examples rendered OK... can you, @mattpap or @bryan or @kdodia, check if 15 seconds in enough for you. My laptop is a core i3 with 4gb of ram, so probably I am a good mirror of travis, :turtle: ja ja ja...
Btw, execute all cells in complex examples take me less time, but this 15/20 seconds include opening the notebook, loading the content, etc., and it seems to take precious seconds, but again, it is probably is my machine...