Allow parallel building

[torsava]

I believe an important feature of the recipes would be to allow parallel building of packages that don't depend on each other.

E.g. for rh-python34, after you build python for the second time (with_rewheel: 1) you can start building multiple packages at once, since many of them depend only on python but not on each other.

This would offer a significant speed-up of the rebuild procedure.

[kasicka]

+1, in rh-nodejs6 after nodejs package is build, the rest 300 packages can be build simultaneously.