Changed completion defaults

If `auto-complete' is installed then that is used by default;
otherwise we use `completion-at-point'.
`C-c TAB' now calls the correct completion engine

It will invoke `completion-at-point' if `auto-complete' is not being used.
Always activate `completion-at-point' completion
Improve formatting of completions for exceptions
Correct doc for `ocp-index-use-auto-complete'