Sbt plugin

See the [new README.md](https://github.com/michalrus/wartremover/blob/f35ec8d0c82fba3e28debdae82bc71472c78da76/README.md) for the details on all the possible ways of using the sbt plugin. =)

@puffnfresh, I've fixed the [build.sbt](https://github.com/michalrus/wartremover/blob/f35ec8d0c82fba3e28debdae82bc71472c78da76/build.sbt) a little, now you can release all three (`wartremover-2.10`, `wartremover-2.11` and `sbt-wartremover`) by doing just `$ sbt "release cross"` (tested with my local maven repo, works seamlessly).