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?
Using addressable 2.2.2 I get the following results:
```ruby
a = Addressable::URI.parse('http:')
# raises Addressable::URI::InvalidURIError: Absolute URI missing hierarchical segment: 'http:'
# a == nil

b = Addressable::URI.parse('::boo')
# raises nothing but returns Addressable::URI::InvalidURIError
# hence, b holds a Addressable::URI::InvalidURIError
```
I'd expect the second statement to also raise an exception but in no case to return one.
I'll go open an issue in addressable.