Added ability to use url as key source

I have a feeling this first implementation is too naive, but I figured I would give it a shot.

Github provides the ability to pull public keys associated with a user via https://github.com/username.keys

I have wanted to use this multiple times, but didn't find it to be possible with the current implementation of authorized_key module. I probably should update the documentation to reflect that a url is allowed, but I wanted to see what the feedback was like, in case I need to change the implementation significantly.