Refactor to bring everything up to express-context 0.3.

Since express-context now provides mixins for free, we don't need to include any chaining of our own. All the existing actions just get refactored as mixins. README updated accordingly.

Will be the first release.
Merge pull request #1 from izaakschroeder/refactor-for-context

Refactor to bring everything up to express-context 0.3.