Add browser script option to package.json

This will tell any browserify project to package the browser version of
mocha and not the Node version.

Fixes issue mochajs#880

This will only cover the JavaScript packaging, CSS is still done
manually through <link> tags.

package.json

@@ -15,6 +15,7 @@
     "url": "git://github.com/visionmedia/mocha.git"
   },
   "main": "./index",
+  "browser": "./mocha.js",
   "bin": {
     "mocha": "./bin/mocha",
     "_mocha": "./bin/_mocha"