Add support for shorttags in functions

[pharixces]

This fixes #1005

[pharixces]

Forgot to run the unittests, will look into it.

[pharixces]

Added an extra interface AttributeFunctionHandlerInterface which extends the FunctionHandlerInterface. The functioncall compiler checks whether the functionhandler implements this interface and sets the attributes of the functioncall compiler. This way you can add your own shorttags (and other attributes) to your functionhandler.

I'm not quite satisfied with the unset($_attr['nocache']); in the functioncallcompiler class. It almost seems this is only added to make the unittests complete for the assign function within smarty. Gonna take a closer look at this.

Edit: took a closer look and now i'm satisfied. (e.g. fixing this would take me too much time).

[wisskid]

this does not change $required_attributes in Base.

resolved it in last commit