Fixes #70

Thanks to Isaku Yamahata.

I've changed the way things work a bit:

1. netns is replaced by mnexec, a general-purpose mininet helper.

2. For interactive commands, we now use mnexec -p, which prints out
   the pid, so we can kill it when someone hits control-C!

3. We close file descriptors for subshells. This might save memory,
   but who knows.

4. We detach our subshells from the tty using mnexec -s; thus
   control-C should not terminate everything.

5. Given 4, mn -c is now necessary if you kill mininet.

- some namespace conflicts due to unpep8
- fixed infinite recursion caused by removing redundant makeIntfPair

'unpep8' script, which does most of the work.

- topo.py is still in pep8
- not all examples work, but this is due to other issues

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.

This required a change to logging, which now uses a singleton pattern.

For all future checkins, 'make codecheck' should pass.