Show keybindings in smex results.

[Wilfred]

The result looks like this:

A neat consequence of this is that you can filter by keybindings, not just command names:

Do let me know what you think. I often forget keybindings and would find this very helpful.

Closes #53.

[ghost]

This is absolutely great and will make me switch to ido + smex !

[adamw]

+1 - exactly what I'm missing from smex :)

[topisani]

same here - as a new emacs user im really missing this - this implementation looks perfect too

[gperaza]

This would be great. The one thing I miss when using smex.