Formats commands.py to use PEP8

I've formatted commands.py to use PEP8 to the best of my knowledge. The most significant things I did were
* Uses 4 spaces for indentation
* Uses "isinstance" instead of "type(x) == list()"
* Uses "is" instead of "=="

This would be a stepping stone to closing issue #37. If you choose to not follow PEP8, please feel free to close this pull request.

Thank you for all of your hard work! I'm really looking forward to using Mailpile :)