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.
Was there a reason to move the type key/value? I intended for the keys to be alpha-sorted here (because in lieu of a better sorting alternative it provides some structure and can make it easier to find stuff). So my preference would be for it to remain in that order.