Add AbstractURL type #127

Per @wsanchez and @twm in #126, consensus seems to be that hyperlink would benefit from an interface/ABC which could be for both DecodedURL and URL/EncodedURL.

AbstractURL is the current name frontrunner, as URL is already taken for legacy reasons.

To quote @twm:

These methods seem to be equivalent between URL and DecodedURL in the sense that they do semantically identical things:

to_text()
to_uri()
to_iri()
normalize()
absolute
scheme
port
rooted

__bytes__ should probably be on this list (per #55). DecodedURL appears to lack it.

I'd also suggest that URL's get_decoded_url() method and DecodedURL's encoded_url property should be part of any common interface. This would be useful in treq to avoid a chain of isinstance() checks.

click() is also similar between the two, though only the DecodedURL version accepts a DecodedURL.

Further discussion to follow, no doubt.