(py-master-file): New buffer-local variable which can be set in the

file local variable section of a file.  When set, and the user hits
C-c C-c, this file gets executed instead of the buffer's file.  Idea
given by Roy Dragseth <royd@math.uit.no>, but implemented differently.

(py-execute-buffer): Support py-master-file variable.  If this names a
relative path, default-directory is prepended via expand-file-name.