Exposing lowess params in regplot, refs #2351

[leoluecken]

This restricts user access to the parameters frac, it, and delta, since I wasn't sure how much of the other parameters would be assumed to hold on the seaborn side.

Also note, that the test test_residplot_lowess_with_kws still fails. This is because the original test test_residplot_lowess does as well.

[leoluecken]

Please note that I didn't test the documentation.
And I just saw, that I accidentally pushed two jupyter notebooks - can you pick these out? (edit: restored them)