Use lodash by default but allow users to pick underscore if they like

Please test this in all scenarios and evaluate if this is something worth merging in. 
IMHO I think it is and I know that @jdalton would appreciate it :)

![screen shot 2014-04-05 at 12 41 55 pm](https://cloud.githubusercontent.com/assets/31543/2623479/085e0b90-bcf2-11e3-8ccc-81a94ed4e35e.png)