Additions for serving versioned files from rails/Automatically untrack deleted files.

First time doing a pull request, my apologies if I'm not up on the etiquette. We're using git to track revisions to csv files that drive the configuration of an application. The modifications I added allow us to serve up a version of a file without changing branches or checking out. I also added support for the -A flag in git add, so that it will handle the removal of tracked files in addition to the addition of untracked ones.

Hopefully these changes can make their way into the mainline in some form. Thanks for developing the gem, it saved us a bunch of time.