Change mappings for F2-F4 to F1-F3.

This frees up F4 for something else, and gets rid of the default :help
binding which I never use anyway.  It also causes F2 and F3 to line up
with the "Undo" and "Redo" labels on my keyboard.
Add CtrlP plugin to allow arbitrary file opening
Merge remote branch 'upstream/master'