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.
The reason I re-ordered the properties is because whenever I was trying to read the schemas, I always found myself having to scroll all the way down in order to read the description of the resource and the information that gives "context" to what is following next: definitions, links and properties. This also makes more sense if you think about it in terms of who writes and maintains  the schemas which is still people given that tooling is not quite there yet.