Bumped dispatch to 0.8.10 so we can release on Scala 2.10.

* Bump sbt to 0.12.4
* Bump disptach to 0.8.10
* Tweak dispatch cross-version selection to work on both sbt 0.12 + 0.13
Use git for versioning.
Fixup dependencies, again.
Create real aggregate project.

* Move plugin into sub directory.
* Create a real aggregate project for the plugin.
Massive build refactorings.

Use moar sbt-isms.  split versoin into sbt file for easier updating.
Merge pull request #41 from sbt/wip/sbt-0.13

Wip/sbt 0.13