Add support for URLs as -p option values.

Try to download any patches specified via the -p option that start with
http or https and apply them.

So this example from README really works:

$ ruby-install -p https://raw.github.com/gist/4136373/falcon-gc.diff ruby 1.9.3-p429
Merge pull request #61 from bkutil/remote_patches

Add support for URLs as -p option values.