Add option to accept a URL to an image

[calum]

No description provided.

[calum]

Support for this may unintentionally already be present:

Jimp.read(lenna.buffer, function (err, image) {
    // do stuff with the image (if no exception)
});

Jimp.read("http://www.example.com/path/to/lenna.jpg", function (err, image) {
    // do stuff with the image (if no exception)
});

[calum]

Confirmed! Already works with urls! This story is now just about adding documentation