Add the required copyright message for underscore code

When looking through the code I noticed that you'd pulled in some code from underscore but hadn't included the full copyright message as required by underscore's license.  Just wanted to tidy this up.

Let me know if I should generate the /dist folder and include in the commit. Also let me know if I need to tidy up the commit any as this is my first time contributing to gridster not sure if I'm following the commit convention guidance correctly.