Also switch from print to info statements, to enable a non-verbose
clean.  Instead of 'mnclean', now run 'mn -c'.

It's possible that mn_clean is a bit easier to read; mnclean may be easier
to type, however.

Added variables to Makefile so that source files are listed in a single
location.

The packages argument was incorrectly specified, leading to an install
process that would appear to have succeeded, but would not actually copy
code.

The error likely occurred due to copying setup.py from another project
where the source files were located in a different position relative to
setup.py.

Also remove usused imports and outdated doc text.

Now, to reference mininet files, use 'import mininet.mininet'.

PYTHONPATH mods are no longer required for installation.