Validate URLs using Addressable gem to allow non-ascii characters (closes #111)

[goulvench]

Warning: this allows a lot more URLs. There is not valid? method on Addressable::URI, because many strings can be parsed and considered valid. See https://github.com/twingly/twingly-url/issues/74#issuecomment-226334749 This change requires a minor version bump, along with a bundle install warning I think.