auto merge of #13063 : brson/rust/dist, r=alexcrichton

Several things here:

* Cleanup
* Fix build targets for building .pkg so that it works and works for all hosts
* Adds support for nightly artifacts
* Put docs in a location suitable for upload to s3 during 'make dist'
* Add coverage of unix binary installers to 'distcheck'
* Fix 'distcheck'
* Change 'dist' to build source tarballs, binary tarballs and OS X packages