Rework javascript api to match README

src/main.js

@@ -0,0 +1,24 @@
+// UMD (Universal Module Definition)
+// See https://github.com/umdjs/umd for reference
+(function (root, factory) {
+  if (typeof define === 'function' && define.amd) {
+    // AMD. Register as an anonymous module.
+    define([], factory);
+  } else if (typeof exports === 'object') {
+    // Node. Does not work with strict CommonJS, but
+    // only CommonJS-like environments that support module.exports,
+    // like Node.
+    module.exports = factory();
+  } else {
+    // Browser globals (root is window)
+    root.returnExports = factory();
+  }
+}(this, function () {
+    var layout = require('./Layout.js');
+    return function(node) {
+      node = layout.fillNodes(node);
+      layout.computeLayout(node);
+      node = layout.extractNodes(node);
+      return node;
+  }
+}));