Add sbt-release & sbt-sonatype for easier releases

I took the liberty of releasing an unofficial Scala v2.11 build of `scala-arm` to [Maven Central](http://search.maven.org/#search|ga|1|a%3A%22scala-arm_2.11%22) (while trying to get a 2.11-build of [`scala-io`](http://search.maven.org/#search|ga|1|a%3A%22scala-io-file_2.11%22)...), so ended up adding this release-based conf.

I used @xuwei-k's [example](https://github.com/xuwei-k/sbt-sonatype-and-sbt-release-sample/tree/single) as a reference.