Simplify Punycode.js usage

See https://github.com/joyent/node/pull/6055.

There is no need to split the host by hand in `url.js` – Punycode.js takes care of it anyway. This not only simplifies the code, but also adds support for RFC 3490 separators (i.e. not just U+002E, but also U+3002, U+FF0E and U+FF61).