Cooler README build status line

Status line is now:
• Very first thing, above the header.
• Trailed by a double-space-seperated, `<sub>`, parenthesized branch name, e.g.: `(master branch)`.
• `<img>` properly vertically aligned with the text.
• All HTML, because of `<img align=…` and `<sub>`, and to make it not look afool.