Update for SBT-Android 0.7

Small update to be able to use SBT-Android 0.7. I also split the configuration in :

  * `build.sbt` is the "visible" part of the configuration, at the root of each project.
  * `project/build.scala` contains the inter-project dependencies and all these tricky build configurations.

It seems like a much better way of managing subprojects to me, but tell me what you think about it!