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 side-stepped this in #175 by avoiding the `open(foo).read` pattern for URIs with `nil` or `file` schemes. We shouldn't be using that on files anyhow, since it will leave dangling file descriptors open. eg:

~~~console
$ pry
[1] pry(main)> $PROCESS_ID
=> 55886

[2] pry(main)> open('/etc/passwd').read
=> [lol snip]

# From another terminal, or after ctrl-z, whatever:
$ lsof -p 55886
ruby    55886 pd    8r   REG                1,3      5253   515836 /private/etc/passwd
~~~