1.2.1~beta2

This second beta for 1.2.1 fixes a few issues of beta1 and adds some preliminary features that will ease further evolution in next versions:

• ~/.opam/config no longer mentions OPAM's patch version, so don't worry about testing the beta and getting back to 1.2.0 anymore.
• Such features include: field libexec: in .install files, field features:, flag verbose and dependency flag dev in opam files, string converter syntax in "filters" in opam files
• Overall OPAM should better support further improvements on these files' formats
• Better message formatting

If you're updating from 1.2.0, 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)