ENH: Enabling (subset) of notebook execution for testing and rendering purposes

[bsipocz]

We should run notebooks with the exception of the slow ones, rather than not running any of them.

[bsipocz]

@connolly - for classroom usage, would you rather have empty notebooks, or executed ones?
(for the website rendering we should have the executed versions, but in the repo we could have empty ones that we run through CI to generate the website (and to test that they still run)).

(With the exception of the chapter9 DL example (and maybe the ones using pymc3), I expect all the notebooks to run reasonably fast for CI purposes if needed)