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?
We should probably just remove this test for now. It's only raising ENOENT if you're not at the exact right spot for the relative path; otherwise it'll raise a parse error.

There's just no way to guard against reading arbitrary files without breaking backwards compatibility, or landing #175 and encouraging users to swap in a stricter loader.