deploy: add a Procfile for heroku

The app still needs to be built with the appropriate buildpack, like
this one:

 https://github.com/mbuchetics/heroku-buildpack-nodejs-grunt
grunt: run "bower install" from grunt
grunt: make "bower install" in Grunt act like outside

We don't want to have a target dir. We use what is provided in the
package. This allows us to get unminified assets while developing.
bower: upgrade to bootstrap 3.2

No other changes seem to be required.