refactor(commonjs): add support for npm and commonjs

Added ui.select to module exports so that after installing via npm, the package can be required in the angular module dependency list such as:

angular.module('myApp', [require('angular-ui-select')]);

This is helpful for Browserify and Webpack.

Closes #1176 #1148 #692

Author: aaronroberson

package.json

@@ -1,6 +1,6 @@
 {
   "name": "ui-select",
-  "main": "dist/select.js",
+  "main": "./index.js",
   "author": "http://github.com/angular-ui/ui-select/graphs/contributors",
   "homepage": "http://github.com/angular-ui/ui-select",
   "repository": {