[Enhancement] Remove Item from Cart if Negative Integers(Negative Item Quantity) detected.

![capture](https://f.cloud.github.com/assets/3077812/2301635/5001aedc-a151-11e3-946c-533dc00ea3fc.JPG)


This is a suggestion to remove the items if the input is negative.
It' doesn't make sense to have a negative quantify item