Responsive drawing example

Further improves the drawing example from the last [pull request](https://github.com/mozilla/togetherjs/pull/933) by making the canvas work properly on all screen sizes. Can be tested by resizing [this](http://fiddle.jshell.net/5f8FL/show/) window. Also works when the users have different window sizes, as the drawings are scaled with the original canvas size 1140x400 as a baseline.

JSFiddle: http://jsfiddle.net/5f8FL/

**Other changes:**
* Init now works by sending a list containing the info of all drawn lines, faster and more convenient than sending the canvas.toDataURL('image/png')-info.
* Clear-button for erasing the entire canvas introduced.


**Screenshot:** 
![image](https://f.cloud.github.com/assets/3536982/1720059/298d6212-61f7-11e3-9021-a14f6f9b4f52.png)