add mkt-prompt element

- Need to work on mobile/tablet

![screen shot 2014-12-18 at 6 05 00 pm](https://cloud.githubusercontent.com/assets/674727/5499226/7038bf22-86e0-11e4-9834-51fe33050cce.png)

@mstriemer @spasovski 
This looks like it is from MDN but should it be wrapped in an `if (typeof CustomEvent === undefined) {`?