Limit completions

This cleans up a little of the code (removes trailing whitespace and a null return) and adds the following limiting functionality:
* Only completions that match the current prefix are returned. This prevents overloading of the completions box which seems to have some arbitrary size limit on it. Overloading it causes completions added by other plugins to stop being displayed (since it is already past that maximum size).
* Completions are now only shown for PHP files. I was having issues where I kept getting Drupal completions in my JS/CSS files as well.