add mkt-prompt element

![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 seems awfully generic. I don't like the close button for `<mkt-banner>`. Maybe we can make a `<mkt-close>` that just emits a `close` event when it is clicked and handles the `X` icon? Seems super overkill but it might be handy.