support for PX dynos

Summary:

* more tests for new and existing stuff
* `index (heroku ps)`, `scale (heroku ps:scale)` and `resize (heroku ps:resize)` all use the Heroku API v3
* `size` can be any string, all the validation is done server side by the API
* the price for `PX` dynos is **hardcoded**. This sucks, but fixing it will require the price to be returned on the API call, similar to what's being done for addons.

/cc @pvh @heroku/api