Improve testing of tutorial notebooks

This is the first step in an overhaul of `ipnbdoctest.py`. Step two will be to release it as a separate package and make seaborn depend on it. It's not quite clean or featured enough for that yet.

As part of improving the testing, I caught and fixed some bugs, and also reorganized the linear models notebook a bit, both for didactic purposes and to improve test stability.

I also added a Make rule to strip the object identifier hexes from the notebook for cleaner diffs. ~~Buuut, this seems to trigger test failures when I run it, so it exists but is not used in the notebooks pushed as part of this PR.~~

~~The tests are also failing because two cells (the one-tailed test corrplot cells) are stochastically nonreproducible. I imagine this has to do with randomness in the correlation permutation test, but I do not understand why it is not reproducible from run to run. (Actually in writing this I had an idea and will be testing it shortly).~~

~~The good news is that the currently failing status actually exercises Travis, which now fail when the notebooks don't reproduce.~~

This is an ugly pull request because getting the Travis build to pass required a lot of debugging of things. But it seems to be good to go. The `ipnbdoctest.py` updates might still be somewhat brittle, but we shall see.