use Underscore.js uniq() helper

http://underscorejs.org/#uniq
http://underscorejs.org/docs/underscore.html#section-53

uniqueArray() looked like about O(n²) time complexity. uniq() should be
O(n).

Another option would just be to use an object as a set.
http://stackoverflow.com/questions/7958292/mimicking-sets-in-javascript
Merge pull request #1 from meonkeys/underscore-uniq

use Underscore.js uniq() helper