Adds a new configure flag, --release-channel, which determines how the version
number should be augmented with a release label, as well as how the distribution
artifacts will be named. This is entirely for use by the build automation.
--release-channel can be either 'source', 'nightly', 'beta', or 'stable'.
Here's a summary of the affect of these values on version number and
artifact naming, respectively:
* source - '0.12.0-pre', 'rust-0.12.0-pre-...'
* nightly - '0.12.0-nightly', 'rust-nightly-...'
* beta - '0.12.0-beta', 'rust-beta-...'
* stable - '0.12.0', 'rust-0.12.0-...'
Per http://discuss.rust-lang.org/t/rfc-impending-changes-to-the-release-process/508/1
