Fix autocomplete with OPAM modules

While the documentation for `ocp-index -I` doesn't make it particularly clear, the default behaviour (locate the OPAM folder and search OPAM packages for autocompletion) is disabled when you use it. I figured you had a reason for making use of it, so I've left that as the default and added an option to disable searching local libraries.

A couple of bonus changes:
* I added a `#` to the line regex to include object method calls in autocompletion.
* ocamlbuild appears to support the `bin_annot` tag out of the box now, using your fix actually passes it twice to ocamlc. I have no idea if this is new in 4.01 but I assume it is. Updated the readme.