Add decorators for callable attributes.

I made the decorators I talked about in issue #270. I don't see any way of closing that my self to merge it with this pull request, so do what you will with it. Maybe it's good to have the issue for proper discussion.

I pulled the meat of the docstrings from the wiki, although I did add a few things, as I think this is the most proper place for that information.

This adds a new file willie/module.py.

I set the tuple form of .rule to raise a DeprecationWarning, as converting to this form is a good chance to stop using that form, if it's deprecated.