Allow choosing URI parser

[skalee]

The Addressable gem features its own URI parser. There are two main benefits of using Addressable:

– its behaviour is consistent among Ruby implementations (and I've already observed differences in stdlib's URI module)
– it conforms to RFC 3986, RFC 3987, and RFC 6570 (level 4) more closely
– it supports URI templates, RFC 6570, and IRIs

Perhaps user should be able to pick URI parser of his choice, like he can do it in Excon.

[ronaldtse]

@skalee I think this is a great idea. Let's do it!

[skalee]

Addressable parser is now a default one. User cannot pick the stdlib's one, so I keep the issue open.