Change organisation in build and setup a way to publish to it on sonatype

[Sebruck]

Since we are now within the census-ecosystem, we should publish new versions to the io.opencensus group-id on maven.

On the one hand, we simply need to update build.sbt with the new orga.

On the other hand we need some way to release new versions to sonatype. To publish signed artefacts to sonatype we use the sbt plugin https://github.com/xerial/sbt-sonatype. Would be great to have some kind of travis job which executes the commands on demand.