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 tried to make this consistent with the rest of the `RemoteUpdate` class, but happy to change it if you believe there is room for improvement.
Awaiting @nulltoken reply to your last comment.