Rewrite of guide-key-mode for emacs. 

Here's a recent picture showing what it does:

[[./img/which-key-bottom.png]]

* Status
It is somewhat stable for me at the moment, but expect to see potentially weird
behavior (then to report it to me!).
** Completed
1. Use idle timers to trigger window popup instead of guide-key's constant
   polling.
2. Add support for replacement lists to modify key descriptions on the fly.
** Ongoing/Planned 
1. Explore alternatives to popwin like window-pupose and using display-buffer
   directly