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?
If this were `{ "$ref": "/etc/passwd" }`, I think we'd still raise a JSON parse error, probably with the contents of the file inside. =\

I'm not sure how we should be dealing with this; maybe just reject absolute paths entirely? That would break backwards compatibility but I don't really know why anyone would be doing that to begin with...