Add dependencies for pushing to Sonatype repository

This adds the necessary bits for publishing to Sonatype (which you can do with the 'release' command).

You'll all need to chat with @obrienm to get set up with accounts on Sonatype. Please edit this pull request to add your details to the 'pomExtra' bit. I've added Jenny to give you an example of what it should look like.

Cheers,
Rob