Add sbt-release & sbt-sonatype for easier releases

I used @xuwei-k's example as a reference for much of this:

https://github.com/xuwei-k/sbt-sonatype-and-sbt-release-sample/tree/single

https://github.com/sbt/sbt-release
https://github.com/xerial/sbt-sonatype
Only add next-version commit /after/ sonatype release complete

...it's more convenient in the case of close-failure.

https://github.com/jsuereth/scala-arm/pull/29#discussion_r11855252
Merge pull request #29 from rtyley/add-sbt-release-and-sonatype

Add sbt-release & sbt-sonatype for easier releases