Add node modules and new code for release (#39)

Co-authored-by: tbarnes94 <tbarnes94@users.noreply.github.com>

node_modules/performance-now/test/scripts/difference.coffee

@@ -0,0 +1,6 @@
+#!/usr/bin/env ./node_modules/.bin/coffee
+
+# Expected output is above 0.005 and below 0.07.
+
+now = require('../../lib/performance-now')
+console.log -(now() - now()).toFixed 3