Added initial bower.json file using existing tag structure

Refs: https://github.com/documentcloud/underscore-contrib/pull/31

This is a basic bower.json file to get Bower in line with more recent versions of the repo. Underscore now has a bower file https://github.com/jashkenas/underscore/blob/master/bower.json

Right now Bower only picks up 2.0 because of the tagging structure.