1.2.1~rc

Release candidate for 1.2.1

This fixes a few bugs of the last beta, and adds a little more flexibility in some parts of the interface (specifying version-control URLs, choosing solver and download commands)

As before, if you're updating from 1.2.0 (or below), you may want to update the jobs parameter in ~/.opam/config to benefit from the new parallelism engine (it used to be disabled at opam init by default)