Added gradle tasks to simplify the release process.

Updated documentation with release process using gradle tasks.

Added gradle tasks:
* gitCreateReleaseCommit creates a release commit from the current
  changelog.
* gitTagRelease tags the release commit.

Also, updated version labels from "gvh_v" to the new style "gvh_version-"
and targeting labels from "gvh_t" to the new "gvh_targets-" to stop
Version Handler from rewriting labels on import.

Change-Id: Iade2fec7ed17bcc0ee8c02108d1a8de2a4f992fa