Make the preview window resizable to better layout testing.

[ernestmarcinko]

• Make use of the jQuery UI resizable component
• The preview window should be an iframe, so that it reflects the actual output

[ernestmarcinko]

Improved preview panel

• instead of an iframe (terrible idea) it uses nested CSS to display the media queried components
• resizable
• small icons to click to select the pre-defined viewport size