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?
That won't help us much, because open can't handle them. However, when we have a custom loader, I think its good when the scheme is properly set.
I wonder why there is no FileURI neither in the URI nor in Addressable.