add validation for the 'items' keyword

### What is the problem / feature ?

The `items` schema field was not being validated against.

### How did it get fixed / implemented ?

Added validation for the `items` keyword based on the json schema spec.

### How can someone test / see it ?

Code?

*Here is a cute animal picture for your troubles...*

![663](https://cloud.githubusercontent.com/assets/824194/4776869/0173201c-5bc7-11e4-807b-9ab1aa717b9c.jpg)