move fitbit API key and secret to .env
add 7 day rolling median line to chart
genericize rolling_median function
specify version of tilt like a good boy
imporve rethinkdb task

experiment with "defensive bash" via [Defensive BASH programming - Say
what?](http://www.kfirlavi.com/blog/2012/11/14/defensive-bash-programming/)
significantly improve presentation of site