Add an app.json
Add a heroku-specific database config
Add Procfile
Use an explicit version range for php

Heroku's PHP installation has 5.6.0RC4 available, which composer will
read as >= 5.3.18 and attempt to use it. We don't support 5.6 yet, so
add an explicit version constraint for <5.6.
Add deploy button
merged changes from Peter Keen for Heroku