Support using INCLUDEJS and INCLUDECSS in twig template format

If we want to keep the INCLUDEJS and INCLUDECSS functionality in the future, then we should be looking into transforming these to Twig syntax as well (so we can get rid of the old template syntax completely in the end).

I propose simply use the Twig include syntax: http://twig.sensiolabs.org/doc/tags/include.html

Something like:

{% includejs('@paybas_testext/scripts.js') %}