Changed read_first_of to return None.
Changed read_first_of to return None.

The read_first_of function now returns None if none of the files listed
could be read.

mdpiper/bmi-babel,mdpiper/bmi-babel,bmi-forum/bmi-babel,mcflugen/bmi-babel,mcflugen/bmi-babel,bmi-forum/babelizer,bmi-forum/bmi-babel,bmi-forum/bmi-babel,mdpiper/bmi-babel,bmi-forum/bmi-babel,mdpiper/bmi-babel,bmi-forum/babelizer,bmi-forum/babelizer,bmi-forum/babelizer,mcflugen/bmi-babel,mdpiper/bmi-babel,mcflugen/bmi-babel