Add Heroku as a provider

As discussed at API Craft, here's Heroku as an OAuth.io provider. It seems to be working according to my local testing, but I'm not sure whether you have a more extensive validation process that you're using.

I've added provider screenshots with the following notes:

* Annotations haven't been added like in screenshots for other providers because the styling of what I'd be putting in wouldn't be consistent with what you already have.
* I've left secrets unblurred for the keys step. These secrets have been invalidated so you can either blur them for consistency, or leave them unblurred; it won't make a difference.

I've added a larger Heroku logo assuming that greater resolution is preferable where possible, but smaller one's are available too if this is only used for the little icons in the admin section.