emacs: don't overwrite pop-global-mark binding

It's quicker than helm-all-mark-rings if I know I only want to pop once
or twice.

Bind helm-all-mark-rings to "C-c C-SPC" (as opposed to "C-x C-SPC") for
when I do not know exactly how many pops are needed.
emacs: enable ace-jump-mode

bind to "C-c j" (j for jump) as opposed to its suggested "C-c SPC".
should train myself to use this a lot, it could be very fast.