Updates and bugfixes to the workshop materials

[rhiever]

Hi all, this PR contains several updates and bug fixes for the SciPy sklearn workshop. Sorry for the huge PR, but this was the easiest way to merge my fixes and updates to the workshop materials.

List of changes:

• Fixed issues #33, #45, #50, #51, #52, #53, #54, #55
• Added pandas as a requirement for the workshop (it's used in some examples)
• Added an aside about scatterplot matrices when discussing the visualization of data. This is a very useful technique to know about for EDA.
• Updated notebook code to sklearn v0.18 compatible code, most notably accomodating the move of many cross-validation modules to model_selection
• Fixed notebook 17 where it was importing a helper plotting method from the wrong directly
• Fixed notebook 17 where it was using %matplotlib notebook instead of %matplotlib inline, which causes issues for some users
• Fixed a known bug in notebook 19 where the widget for the decision tree was not displaying the generated tree when the user set the max_depth > 0
• Fixed notebook 20 exercise to properly generate the XOR data (+noise features)
• Various typo and grammar fixes

[rasbt]

Wow, thanks a lot for all the bug fixes. Also great that you added the missing exercises! That will come in handy for whoever needs to prepare the SciPy 2017 workshop :). We will just have to bit careful how we merge since I just saw that it replaced SciPy with "Webstep" at various verious points.

[rhiever]

Made a couple fixes per your suggestions.

[amueller]

we should definitely get travis up for this. I just moved jobs yesterday and I'm pretty busy :-/

[amueller]

Ok, I'm merging this... thanks!

[rhiever]

Added an issue about Travis CI.