Support resizing in the ps:scale command.

WIP: This causes test failures because the API is stubbed out in a weird
way I haven't been able to figure out.
Fix a bug where relative scaling amounts weren't checked.
Mention scale+resize in help text.
Fix specs.

For some reason using an integer works fine against the real API, but
against the mock it needs to be a string.
Update help text to include resize output in ps:scale.
Merge pull request #950 from heroku/resize-in-scale

Support resizing in the ps:scale command.