Imageshare example

I've added another example - you upload a file and it sets it as the other browsers background.  Shows users how to a) read from an input field and b) broadcast to other users (though maybe there's a better way to do that).

This (kind of accidentally) contains the code in binaryjs#47

I've added some hints on the file input for a phone browser to kick up a camera, so you end up getting stuff like this:

![1374255566079](https://f.cloud.github.com/assets/51385/865272/583c1ce4-f635-11e2-8ff9-1d4ad3587e21.jpg)