Add sbt-cppp

**sbt-cppp** (**Sbt** <strong>C</strong>ross-<strong>P</strong>roject <strong>P</strong>rotobuf <strong>P</strong>lugin) is a [Sbt](http://www.scala-sbt.org/) plugin to support [Protocol Buffers](http://code.google.com/p/protobuf/), especially in multi-project builds.

https://github.com/Atry/sbt-cppp/