separate completion list and help into separate popups

NOTE: not quite ready for merge, as I haven't yet accommodated code handling correct positioning of help.

![popup](https://cloud.githubusercontent.com/assets/1976582/5080524/2cad7626-6e76-11e4-94c2-5e8a2c8d8f04.gif)

@jjallaire: any thoughts on improving the styling for the help popup?