Using cython in pandas tutorial
### Please see #3965 for current draft.

Re: this topic https://groups.google.com/forum/?fromgroups#!topic/pydata/aLxALYqosOU (cc @jreback).

I think this is one of the killer features of pandas so I think merits being in the docs. (I'd definitely be interested in reading it!) Maybe something structured like this with a good example:
- write in python first (unit-test, and check for speed, it may be good enough!)
- try and rewrite in python to be more efficient (now, it may be good enough!)
- profile to work out which part is slow (and needs cython love)
- writing and calling a cython function (to do that slow bit faster)

I'm not sure what would make a good toy example for this (and I think that choosing a good one is crucial).

Thoughts?
