Chrome frame awareness

This change allows running IPython Notebook on unsupported IE browser, if user has installed Chrome Frame. This header has no effect in other environments. More info: [FAQ](http://www.chromium.org/developers/how-tos/chrome-frame-getting-started/chrome-frame-faq#TOC-For-Developers)

<a href="http://i.imgur.com/tSQYB.png">![img1](http://i.imgur.com/tSQYBs.png)</a>

It's possible for user to create custom and lighter IE COM object, and render IPython Notebook inside:

<a href="http://i.imgur.com/78eCe.png">![img2](http://i.imgur.com/78eCes.png)</a>