OPTIM use C implementation of the Pickler

This is only possible under Python 3.3+. Under Python 2.6 to 3.2,
we are still limited to the pure Python pickler as the C compiled
pickling system cannot be customized on a per-instance basis.

This is a fix for scikit-learn/scikit-learn#3835.
DOC update changelog
Merge pull request #173 from ogrisel/optimized-pickler

[MRG] OPTIM use C implementation of the Pickler