Converted README HTML badge code (back) to Markdown

All the HTML in the README.md for the badge code was starting to get to me.

Converted to the closest approximation in Markdown image/link constructs, giving up the square brackets in the alt tags and image alignment (“(master branch)” text now `<sup>` instead of `<sub>` to sorta line up).  Kept `<br/>` tags because they provide nice visual distinction.