add Excon::Utils

There may be other methods besides `port_string` that could be moved here.
But for now, I'm more concerned about allowing Webmock to determine valid request keys
using a method call we can control, given the upcoming changes in #289.
I'll update my Webmock PR for that once this change is versioned.
It would be nice if that was in a Webmock release the next version of Fog could require to eliminate those warnings.