Adds GitHib release badge.

This PR also fixes the github tag badge.

Note: At the moment, there's no obvious difference between the two types of GitHub badge. I've also used one of my own repos for this one because I couldn't find any others with releases. If you know of a higher profile repo using releases, I can update this PR with it.

I could be being presumptuous... I've taken the liberty of adding my gravatar to the contributors too.

Closes #142