R completion help + signature tooltip

This PR modifies the help popup, and brings in a signature tooltip, as part of the autocompletions.

![popup](https://cloud.githubusercontent.com/assets/1976582/5150232/5b1e3966-7185-11e4-8a6e-1c76536d93e5.gif)

Main bits to notice:

1. The completion list and help are now separate popups, and resize independently (although they align at the top).

2. The signature tooltip stays on-screen while within the function call -- we track paren insertion to determine when the user has left the function call (or escape on mouse clicks, `{`, etc.)

@jjallaire, you mentioned that you would be willing to take this work and then refactor it into a common display for the C++ popup -- any other comments re: styling / display here?