[WIP] Data independent CV and model_selection module

Hi guys,

I'm working on #1848 and #2904, and would like to get some feedback on my changes.

Here's a writeup about the issues/changes: http://pignacio.github.io/jwoc-blog/stories/stuff-about-scikit-learn18482904.html

Any thoughts / suggestions / improvements / rants on the changes are greatly appreciated
Please also import public classes from other modules. (I expect you had planned to do so, but elsewhere you have TODOs for similar things.)