Auto merge of #3345 - alexcrichton:release-branches, r=alexcrichton

Add support for release branches in Cargo

Follow the same strategy as the compiler for now in basically every respect:

* Add new `--release-channel` configure option, defaulting to `dev`
* Remove old `--enable-nightly`
* Add `--enable-build-openssl` as an orthogonal option
* Hook up Travis/AppVeyor to stable/beta/master branches to do the right
  channel builds.