Drastically simplify distribution building for 2.11.

See README and individual commits for info.

The main idea is that these scripts should do nothing more than repackaging org.scala-lang % scala-dist and its transitive dependencies. No duplication allowed. The process is reduced to pushing a tag to scala/scala-dist by the same name as the one from scala/scala (whose artifacts have been published to maven), and running `sbt s3-upload` on windows (for the msi) and unix (everything else).

We no longer package source code ourselves. GitHub provides source archives for every tag.

Review by @jsuereth, /cc @retronym, @gkossakowski.

This has all been tested locally. I'll do a dry run on jenkins tomorrow and link to the results.