add Cython version as a pip dependency
When I tried to pip install pandas on a system with cython 0.15.1, it failed (I'm guessing that's not surprising). After pip installing cython (to 0.22), pandas installs fine. The error messages talked about missing lib.pxd and tslib.pxd files - to my untrained eye, Cython wasn't an obvious culprit!

My point is that, unless there's a good reason not to, the minimum cython version for compiling pandas should be a dependency in pip.
