!rem, clu, mul #3154 Upgrade protobuf to 2.5.0 and add an SBT task

* Added an SBT task to compile protobuf files with protoc
* Upgraded protobuf to 2.5.0 which is the current stable
Merge pull request #1724 from akka/wip-3154-upgrade-to-protobuf-2.5.0-ban

!rem, clu, mul #3154 Upgrade protobuf to 2.5.0 and add an SBT task