observe the size attribute; save qr codes for rendering again later.

First, thanks for writing this directive. It's very cool.

My application needs to resize QR codes on the fly. So I started observing changes in the size attribute and rerendering. I rewrote the render function so that it doesn't require a new `qr` be created on every render.

I didn't want to change the index.html, since that demo is on your gh-pages site. So I put together a demo of resizing on another branch ([demos/observe-size](https://github.com/bmatsuo/angular-qrcode/tree/demos/observe-size)). It's pretty rough but it actually looks pretty cool when you are way zoomed out.