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?
I disagree; URI::File is the wrong direction to go in. We only use it so that we can keep using open-uri, but we _shouldn't_ be using open-uri to open local files -- there are *many* more obvious means of doing that (eg, File.read, Pathname#read, etc etc).

We can assign schema URIs as `file:///tmp/ohai.json`, but when we _read_ that file, it should be using `File.read(uri.path)` or something similar.