DOC: Update docs to reflect actual version support.
I don't think this is true anymore (from install.html on pandas site)

> Officially Python 2.6 to 2.7 and Python 3.1+, although Python 3 support is less well tested. Python 2.4 support is being phased out since the userbase has shrunk significantly. Continuing Python 2.4 support will require either monetary development support or someone contributing to the project to maintain compatibility.

I'm pretty sure that 2.4 (and 2.5) will fail, if only because the `from __future__ import with_statement` have been removed from many files.  I also think we're at the point where we can say that Python 3.1+ support is actually there (this could be a change in docs for  v0.13)
