Add Grunt-related instructions to the README
Grunt: Add git-describe task, and add version info to banner

The git-describe tag isn't very useful right now. There have been
1443 commits since the last tag, which looks like a quickcompose
release.
Grunt: Add new 'grunt sizes' task that outputs file sizes

Shows the file sizes for foam.js, foam.min.js, and gzipped sizes of
same. The last, foam.min.js.gz, is the most important to make small.