Add an entry point in package.json

This adds a file, `index.js`, which loads all of the mixins and exports
the modified underscore library. After setting the `main` property in
package.json to this index.js file, underscore with the contrib modules
mixed in can be included a project by doing:

`var _ = require('underscore-contrib');`
Merge pull request #25 from brianloveswords/node-module

Add an entry point in package.json