Bump to sbt-pgp 0.8
Cleanup error messages and stub support for Ivy/Sbt plugins.

* Split out certain bintray tasks into their own keys
* Ensure error messages are displayed when, e.g., licenses are bad
* Start to prefix keys with `bintray` to avoid naming collisions
* Stub in a new SbtPluginBintray{Resolver,Repository}.
* Move dependencies of tasks into `publishConfiguration`
  rather than `publish`.  Sort of a step one for being
  friendly to sbt-pgp plugin.

Still waiting on a new version of bintry from @softprops, so we
can use the upload method to push ivy artifacts.
Merge branch 'master' into wip/publish-sbt-plugins
snapshots
optional settings for bintray organization
more work towards publish to non mvn repos
better defaults when project is an sbtPlugin
release notes for next release