BUILDR-645: Support Mercurial for 'release' task.

Add Mercurial support for releases.

Based off the Git and GitRelease classes. The only major difference is that there is no equivalent of "git remote". Instead, I used "hg paths" to find either the default or default-push remote path.

Reference:
* http://hgbook.red-bean.com/read/a-tour-of-mercurial-the-basics.html#default-locations