Added support for `completion-at-point'.

Added a customizable variable `ocp-index-use-auto-complete'. When set,
it gives the previous behaviour. If unset, the standard
`completion-at-point' mechanism is used.

With this change, `auto-complete' package is no longer a requisite to
use `ocp-index.el'.
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'
Merge pull request #32 from nojb/master

Added support for `completion-at-point`.