BLD: Cache 2to3 artifacts

This is the logical followup to the cython cache hack.

Using both, I put together a server on Redhat Openshift to serve as 
network cache storage, and a travis build now takes 1.5 minutes bare,
and up to 4 minutes for a FULL_DEPS build.

The traffic used up by each build is significant, so the travis bit can't
be put on master. the 2to3 cache will shave another couple of minutes 
off local iterations, though.

https://travis-ci.org/y-p/pandas/builds/5873760