title: set width and height of canvas in fabric.js
body: <p>I have an image background in my HTML5 canvas. </p>
  
  <pre><code>var canvas = new fabric.Canvas('#canvas1');
  canvas.setBackgroundImage(
              'http://fabricjs.com/assets/jail_cell_bars.png',
               canvas.renderAll.bind(canvas)
         );
  </code></pre>
  
  <p>How to set dimensions of my canvas (width, height) to the dimensions of the background image?</p>
  
  <p>Thanks.</p>