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');`

(Note, you will also have to publish the module to NPM before the above
will work.)

I made the assumption that for the node module "just mixin everything"
was a valid strategy. I'm happy to change it if that's not the case.