Prevent cookbook negativity (when specifying items and start params)

:fork_and_knife: If the items or start parameters are negative render a 400 with a helpful error message.

![screen shot 2014-08-04 at 12 18 11 pm](https://cloud.githubusercontent.com/assets/316507/3799941/0825187a-1bf3-11e4-87e8-f3488d4a487e.png)