chore(commonjs): Add support for commonjs and npm

[kevinboosten]

Added angularjs-dropdown-multiselect 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('angularjs-dropdown-multiselect')]);

This is helpful for Browserify and Webpack.

[gpsimmo]

Is there any update on whether this can be merged. Bower installations are really deprecating.