Revert mvn group-id to io.brooklyn.networking

- We can’t change the maven group id of this repo to 
  org.apache.brooklyn.networking because it is not an apache project.
  Apache own that domain, so we can’t push to sonatype with it.
Merge pull request #4 from aledsage/fix/revert-maven-group-id

Revert mvn group-id to io.brooklyn.networking