SPARK-4466: Provide support for publishing Scala 2.11 artifacts to Maven

The maven release plug-in does not have support for publishing two separate sets of artifacts for a single release. Because of the way that Scala 2.11 support in Spark works, we have to write some customized code to do this. The good news is that the Maven release API is just a thin wrapper on doing git commits and pushing artifacts to the HTTP API of Apache's Sonatype server and this might overall make our deployment easier to understand.

This was already used for the 1.2 snapshot, so I think it is working well. One other nice thing is this could be pretty easily extended to publish nightly snapshots.