Support for Mercurial and Subversion

First off, thanks for writing GitGutter. It's a great addition to my coding workflow.

I've extended GitGutter to support Mercurial and Subversion in addition to Git. I've refactored and renamed a number of files to generalize the functionality common to all three version control systems. Otherwise, the necessary changes were quite straightforward. I also changed the name to VcsGutter to reflect the broader support.

I'd understand if you're not interested in supporting these additional VCS's. If so, I'd like to continue to maintain my fork and publish through Sublime Package Control.

If you would like to accept this pull request, I'll gladly lend a hand with any future Mercurial and Subversion support that is required.