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?
For the second: It depends I guess. File uris are a problem in general so far. open does not like the file:// scheme nor escaped paths. I agree for the parts that are actual URIs but we don't now that before e.g. nested refs have been resolved against their parent scheme's URI.
So I wouldn't do much about this until we decided how to handle files.