README should use Markdown format

Hey Kyle,

I think `libfitbit` is badass, but do think it'd be better for the README to use Markdown rather than AsciiDoc format. It's mostly a superficial change, but this is how the main repo's README looks on Google Chrome.

![openyou_libfitbit GitHub](https://f.cloud.github.com/assets/172692/62694/1ab57118-5d51-11e2-94b0-c5c611e60464.png)

Thanks for putting so much time into the project. It really is awesome.