Adding automation for releasing *and* directly release to bintray.

* Create helpers to ensure appropriate bintray setup.
* Add mechanism to acknowledge (via REST) that a release is complete and good
* Create release script to automatically tag/test/push packages and remember
  all the steps to actually ensure a bintray release is coherent, complete
  and basically working on linux.  Windows can suffer as usual until we
  have more infrastructure.
Adding documentation how to map directories
Merge pull request #160 from sbt/wip/directory-mappings-docs

Adding documentation how to map directories
Merge pull request #157 from sbt/bintray-releasing

Adding automation for releasing *and* directly release to bintray.