Adds a Url property to the RemoteUpdater, which allows the remote url to...

... be updated.

This is a PR for issue #788 

Includes a single test case for verifying that the remote url was indeed changed.
Let me know if anything needs to be changed or corrected for this PR.
I see this remark exists on the `RefSpecs` properties, but not on `TagFetchMode`. I really don't know what it means... any existing `Remote` instances will not change, but the one returned from `Remotes.Update(…)` will. Thoughts, @nulltoken?