Use Addressable gem to handle URIs

This would cover #173 
It adds support for Asian URLs.
When I changed the validation for the URI format to Addressable, I thought that the value should not be escaped (it was introduced to establish compatibility with Asian URIs by eca92afb219791b928426c95b2556693db82b879). The value either is parsable as URI as is or not and in the latter case no valid URI.
What do you think?
Yeah this is an interesting  test... I think it has to be a JSON file to test for potential attacks (as @pd said) and this won't work on windows (even if the relative path was correct) - maybe it should link to a Json file that's already part of Json-schema? One we know exists? (Maybe test that the file exhausts as a precondition).

I think we shouldn't break backwards compatibility just yet :unamused: