Add support for Emacs-style prev/next keybindings.

Among other places that support Ctrl-P and Ctrl-N for "prev" and
"next" line movement, Mac OS X supports this systemwide, so it's
nice to recognize them like arrow keys in jquery-textcomplete.
Merge pull request #57 from metamatt/mg-emacs-arrow-keys

Add support for Emacs-style prev/next keybindings.