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.
What `prmd init` generates is just a basic starting point, one has to anyway modify it to better fit specific needs. 

I would say, let's generate according to what the HTTP API Design Guide suggest and clarify in the prmd's README that the scaffolding is only suggested and that it is intended to be modify and completed by the developer.