Both commands enable handling of a little known feature
of git, which is to support multiple URL for one remote.
You can add multiple url using the `set_url` subcommand of
`git remote`. As listing them is also handy, there's a
nice method to do it, using `get_url`.

* adding set_url method that maps to the git remote set-url command¶
 * can be used to set an URL, or replace an URL with optional positional arg¶
 * can be used to add, delete URL with kwargs (matching set-url options)¶
* adding add_url, delete_url methods that wraps around set_url for conveniency¶
* adding urls property that yields an iterator over the setup urls for a remote¶
* adding a test suite that checks all use case scenarii of this added API.¶

Signed-off-by: Guyzmo <guyzmo+github@m0g.net>