Switch to company mode

Unfortunately switching to company mode from auto-complete is mixed in with some other commits of me tweaking my own settings.

This switches to company for autocompletion. It is what the maintainer of cider uses and recommends. It does not currently have support for documentation pop-ups.  As a result the `C-d C-d` shortcut has been removed in favor of the default cider-mode `C-d C-d d` (though it brings it up in a new buffer).

I've had a a mostly better experience with company than I've had with ac-cider lately.

http://company-mode.github.io/