Supervisord export tests, 

This proposed PR adds:

  + A fix to use the shell provided in parameters in supervisord export (defaults to `/bin/sh` in `honcho/command.py`)
  + The fix proposed in #65
  + Basic tests for supervisord, specifically for those two changes

Let me know what you think. Happy to add code styling changes, additional tests, etc. 

Honcho has been very helpful to us here, thank you for that, and here's to hoping these changes are useful to you.