Slight modifications for jam.js support (partially fixes #79)

I told jam the package name is "hbs" so that the plugin is hbs!, and I rewrote the dependencies to use relative urls. The reason It's only a partial fix because there's currently no way to configure it for jam compile at the moment - see https://github.com/caolan/jam/issues/127

(And, of course, you'd need to create a jam account and publish it.)

I kept the dependencies included in the package, although we could update it to use jam-provided coppies of underscore and json2 (and possibly handlebars) if we wanted.