Added new aliases and plugins for git

New Aliases:
* gus - Unstages the passed file ex. `gus aliases/available/git.aliases.bash`
* gm  -  Merges passed branch into current branch ex. `gm git-additions`
* gcm - Commits with the passed message `gcm "Added new aliases and plugins for git"`

New Plugins:
* git_pub - Publishes the current branch to the remote origin ex. `git_pub`
* add_branch - Adds and tracks the passed branch name from remote origin ex. `add_branch git-additions`
* git_revert - Reverts **all** commits back to the passed commit on top of HEAD (ie. doesn't change history)  ex. `git_revert 1a885b957f`
* git_rollback - Resets the current HEAD to the passed sha (includes option to reset --hard) ex. `git_rollback 1a885b957f`

*Note - Resetting the HEAD can be scary/confusing for new Git users, I've found this `git_rollback` plugin eases some of those fears*
*Thanks to [this SO answer](http://stackoverflow.com/questions/1895059/revert-to-a-commit-by-sha-hash) from Charles Bailey for the necessary commands for the `git_revert` plugin*