Add grunt task

Adds a grunt task so that projects using grunt may run lumbar. Example usage (documented in the task itself):

    var port = 8000,
      publicDir = './public',
      lumbarFile = './lumbar.json';
    
    grunt.loadNpmTasks('lumbar');
  
    grunt.initConfig({
      server: {
        base: publicDir,
        port: port
      },
      lumbar: {
        // performs an initial build so when tests
        // and initial open are run, code is built
        build: {
          lumbarFile: lumbarFile,
          outputDir: publicDir
        },
        // a long running process that will watch
        // for updates, to include another long
        // running task such as "watch", set
        // background: true
        watch: {
          background: false,
          lumbarFile: lumbarFile,
          outputDir: publicDir
        }
      }
    });
  
    grunt.registerTask('default', 'lumbar:build server lumbar:watch');