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.
One thing to add to this discussion is that the default value of this timeout setting could be configurable depending whether tests are run on Travis CI or not. This is what I did for `--timeout`, which is used for allowing to stop examples on Python-level. Grep for `TRAVIS_PYTHON_VERSION` in `examples/test` for details how this works. It's still insufficient, but at least locally examples run fast and on Travis CI they rarely fail spuriously. Thought the situation is a little different here, because only interactive/animated examples always take the entire time interval, which in case of notebooks will happen for all examples.