Rearranged things so that compilation of .py files is the responsibility

Rearranged things so that compilation of .py files is the responsibility
of the 'install_py' command rather than 'build_py'.  Obviously, this
meant that the 'build_py' and 'install_py' modules had to change; less
obviously, so did 'install' and 'build', since these higher-level
commands must make options available to control the lower-level
commands, and some compilation-related options had to migrate with the
code.