Add support for Mercurial

Hello,

it would be very useful if the buildnumber plugin also supported Mercurial.
I'm attaching a patch that adds a new goal to the plugin called 'hgchangeset' that works in a similar way to the current mojo. I kept the implementation simple (not very flexible, just suits a basic need). The goal sets two project properties with the changeset id and changeset date.

Thanks.