Prepare 3.1 release.
Some bugs are blocking users in 2.stable and the code diverted enough to
warrant releasing 3.1 rather than spending time fixing 2.stable. Also, it's
been a long time without release and we have some excellent new
features (dependency support, el-get-list-packages, etc).
The el-get recipe is already refering to the 3.stable branch, that will be
created from this commit.