support for PX dynos

These changes depend on a new version of `heroku-api` (heroku/heroku.rb#80).

I added support for non-numeric dyno sizes. `P` is the only non-numeric size supported for now.

Highlights:

* more tests for new and existing stuff
* `size` is always a string to/from the API
* sizes never contain the X (as in `2X`, `1X` or `PX` /cc @csquared 
* `P` is hardcoded as the only non-numeric size the CLI will accept
* invalid sizes cause `ps:scale` to fail silently. When there are multiple types being scaled at the same time and at least one of them is valid, the invalid ones are ignored. It was already like this and I decided not to change it here. I think that a new GH issue would be more appropriate.
* fixed a small inconsistency between `ps:scale` and `ps:resize` with lowercase Xs (eg.: `2X vs 2x`)

/cc @pvh @heroku/api