Move to python 2.7.6.
Pickup xz changes from vincenzo.
Misc root changes.

- Pickup multithreading fixes for reflex by Chris.
- Pickup roofit patches to cms-sw/root and drop them from here.
Merge pull request #789 from ktf/merge-next-into-stable

Merge next into stable
Merge remote branch 'origin/IB/CMSSW_7_2_X/stable' into IB/CMSSW_7_0_X/stable