Allow relative paths for --listener and --executorFactory option

Make use of _node's_ `path` module to resolve the given _path_ for `--listener` and `--executorFactory` option before the attempt to `require` them as modules. This should fix Zarkonnen/se-interpreter#10.

Maybe you want to have a look.

This request also includes a few fixes for some _jshint_ issues (in a separate commit), namely

  + trailing white-spaces
  + semicolon related
  + _strict_-equality

regards
~david