Create a Stage class

[jmgate]

Given the way the conceptual stage is implemented in the code, it'd be real nice to have a Stage class, with methods defined for the different phases, that users could inherit from in creating their own stages. We didn't do things this way initially, because we wanted the ease of a user to be able to simply apply the @stage() decorator to a method, and have StagedScript handle all the rest of the magic behind the scenes. That said, it might be possible to preserve backward compatibility and have a decorator dynamically create a Stage derivative behind the scenes. I don't know. I get the feeling this would clean some things up—especially for power users—but at the expense of a higher barrier to entry for newbies.

Note: Depending on how this is implemented, it'll likely be a breaking change.