Add a bug fix plus tests for calling functions in Jinja via dot-lookup notation

@thatch45 does the ``abspath`` change in loader.py tickle you the wrong way? This fixes I problem I see occasionally when invoking Salt's loader at the Python-level. It is due to ``self.module_dirs`` in the loader class being set to ``salt/modules`` or ``salt/renderers`` instead of ``/path/to/salt/modules`` or ``/path/to/salt/renderers``.