Add support for alternate github repos

New feature time! This is of relatively limited utility for most folk, but figured I'd try to get all my commits in. I added support for alternate github repos (namely, me for when I'm doing my testing) in an easier fashion, by having an UPDATE_URL variable that points to the github repo/branch. The commits all point to the correct one for msm, https://raw.github.com/marcuswhybrow/minecraft-server-manager/master, but it's nice to have the option to change in one place for folk who are doing their own development and want to iterate on their own repos. (or try out alternative branches).