Heroku button

NOTE: this includes the commits in #812 because i couldn't get it to work without them.

This adds a ["Deploy to heroku"](https://devcenter.heroku.com/articles/heroku-button) button to the README; I successfully deployed a new instance of rstatus from [the button on my branch](https://github.com/carols10cents/rstat.us/tree/heroku-button).

This creates a free rstatus instance in minutes with very little info needed from the end user (if they already have a heroku account, and assuming everything works).

I went with the most bare-bones configuration-- no twitter integration (we really should disable all twitter related buttons when those env vars aren't set), no elastic search. Those could be added without increasing the cost, but do add complexity. They certainly could be added later but hey, MVP!