Add support for publishing to Maven Central

On order for this to work, sbt-pgp must be set-up correctly and the Sonatype OSS
credentials need to be placed in ~/.sbt/0.13/sonatype.sbt.

Please see http://www.scala-sbt.org/0.13/docs/Using-Sonatype.html and http://www.scala-sbt.org/sbt-pgp/index.html
for details.