Minor cleanup

[delirious-lettuce]

@wessilfie ,

I was just reading your article on twilio and thought I would take a look at this repo. I noticed a typo (greatful -> grateful) while reading through app.py which this PR fixes plus a bit of cleanup too!

• Group imports (standard, third-party, local)
• Use os.environ.get with a default value in case of no token(s)
• Two blank lines around top-level definitions
• One space after # character
• Use docstrings where applicable
• Use consistent 4 space indentation (else clause of receive_message)
• Fix typo: greatful -> grateful