recipe/promise_plugin: Improve options

Refactor promise_plugin recipe options:

• class: name of the class implementing the promise
• module: dotted module path from which to import the promise class
• file: file to exec which allows a promise to be implemented outside of an egg

The class and module option are just a cleanup/refactoring of the previous content or import option.

The file option is new, it's an alternative to module that allows a promise class to be implemented in a standalone file (as opposed to an egg) and "included" in the plugin via exec instead of imported from an egg.