Syncs up with Heroku's platform API schema and HTTP API Design Guide

* Makes sure to use the interagent meta-schema as the current Heroku's platform API does.
* Adds `stability` and `strictProperties` to resource schemas
* Fixes `identity` property so that it follows the HTTP API Design Guide
* Re-orders properties to make schemas more readable for humans.

Hopefully this will close #186 and #185. All tests are passing.
Fair. We certainly call it out as something worth supporting, so having it seems alright. Thanks for discussing.