Added resize option to preview container

I've added the resize option also to the preview div. This is a new css3 feature which is not supported by all browsers, currently only firefox 4+, chrome 4+, safari4+ (they support also resize on divs), opera 12.1+ only supports resize on textarea's, but support should come

see: [Can you run it: Resize](http://caniuse.com/css-resize)

So this is a feature what will eventualy be supported in all browsers, but it is a nice-to-have in the current browser who support this!