Allow-post-build tagging of multiple branches

This extends the master/CURRENT tagging system to cope with multiple branches. After a successful build, any branch that appears in `tagBranches` will be tagged with `{branchName}-{buildTime}`, and with the tag name specified in `tagBranches`.

This changes the ongoing tag naming from `release-{buildTime}` to `{branchName}-{buildTime}`. I'm not 100% sure this is even necessary any more, to be honest.