Sbt plugin

See the [new README.md](/michalrus/wartremover/blob/f8d261ae24101c119debf0a6f947acb52ce163e0/README.md) for the details on all the possible ways of using the sbt plugin. =)

@puffnfresh, I've fixed the [build.sbt](/michalrus/wartremover/blob/f8d261ae24101c119debf0a6f947acb52ce163e0/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 own maven repo, works seamlessly).

All vals representing built-in warts in `wartremover.Wart` object in `sbt-plugin/` are auto-generated from `core/` source at `sbt-plugin` compilation (see the new `build.sbt`).

I've also fixed the shell script, it's now using `"$(dirname "$0")"` as a prefix to JAR's path. It should be something like `readlink -f $0`, but that does not work well with OS X.