load from url

*Depends on #619, pull it first.*

Adds support for `.load(url, cb)` which streams an html document from the web into the parser. The callback receives `(err, $)`.

Sample program:
```js
var cheerio = require('./');

cheerio.load('http://example.com/', function (err, $) {
  if (err) { throw err; }

  console.log($('h1').text());
});
```