Git Remotes: Adding and Deleting URLs

Git has the ability to adding and deleting multiple URLs for a single remote.  Multiple URLs are useful for deploying code to multiple servers.

`git remote set-url --add [name] [url]`
`git remote set-url --delete [name] [url]`
http://git-scm.com/docs/git-remote

This pull request adds the ability to add a remote URL and delete a remote URL for a specified remote (by name).

The function names could be tweaked, if there's a strong opinion.  I considered being more verbose, with `remote_set_url_add` and `remote_set_url_delete`, but thought that the more simple names were better.