Added the ability to load via require

Prior to this change, we had to use script tags to load
playground.js. Now, we can simply download playground using
npm, and use CommonJS' `require` to load the module.

This commit adds a `package.json` file for the sake of compatibility
with npm, a `.gitignore` file to ignore the `node_modules` folder,
enhances `plugins/playground.three.js` to be loadable via `require`
and enhances build.js to distribute a CommonJS-equivalent of the
build files.

.gitignore

@@ -0,0 +1 @@
+node_modules

build.js

@@ -62,4 +62,9 @@ for (var key in builds) {
 
   output.to("build/" + key);
 
+  output = 'var PLAYGROUND;\n' + output;
+  output += '\nmodule.exports = playground;';
+  output += 'playground.Application = PLAYGROUND.Application;';
+  output.to("build/commonjs/" + key);
+
 }