The convention is that sys.exit(...) is called only if a module is run as a script,
and that main() entry points use return statements to report exit codes: 0 (or None)
for successful execution, or any non-zero integer for errors.
E.g. see the console scripts generated when installing with pip.
ttx is now equivalent to:
$ fonttools ttx
pyftsubset can be called as:
$ fonttools subset
varLib can be called as:
$ fonttools.varLib
Also adds a executable at toplevel called fonttools, such that
with "./fonttools ..." one can run stuff without installing.
Python seems to automatically include ./Lib into its search path.
