Support to obtain the image in an object (serializable in JSON format)

I don't know if it's really useful, but, with these commits, it's possible to obtain not only an `<img>` or a `<table>` tags but also an object (serializable to JSON in a HTTP response) whose properties are the attributes of the `<img>` tag. It could be good for generating Backbone.js Views, for example. In order to do this, a new function was added: `qrcode.createImg(cellSize, margin)`.

Also, there's no need to use the `base64EncodeOutputStream` function, as it can be replaced with a `Buffer` class. I thought it could lead to a performance improvement but finally, it couldn't.

I specially like this module because is independent from other modules such as node-canvas that can be a pain in the ass.