Add async implementation - simplify using file-manifest.

Adds an async implementation. While you're right that this is not _absolutely necessary_ (since you're probably generating the html in some sort of build process where speed is less critical), with `file-manifest`, it's not significantly harder to do. This way, all your markdown files can be processed "simultaneously" (roughly), which should speed it up (especially if you have LOTS of posts).

Additionally, what do you think of implementing a _real_ test suite. I personally prefer `mocha.js` but have used jasmine as well.