Heroku deploy button

Well, this does indeed add a Heroku Deploy button, which will make it easier to try out Factor.

It really just adds an app.json and the button to the README. This is not the cleanest PR, so no worries if you just lift the ideas instead of accepting.

I can't get the darn thing to work due to issue #1, so before this is accepted, a bit of Factor debugging might need to happen. :)