README Cleanup

@nseidle - Here's my attempt at readme cleanup for this repo.

This one was easy - port to markdown, add images, links, and headings. The license information and version history was already very well defined. It may be worth eventuallky adding a "Repository Contents" section ([see the similar pull req on the ColorLCDShield for an example](https://github.com/sparkfun/ColorLCDShield/tree/readme_cleanup)) because there's a lot in this repo, but that wouldn't be necessary for this branch.