Piper/add header validation

### What is the problem / feature ?

Validation for request headers was not implemented.

### How did it get fixed / implemented ?

Added validation for parameters who's `in` value was `header`.

### How can someone test / see it ?

Validate a request against a schema with headers.

*Here is a cute animal picture for your troubles...*

![4909924330_1a47ffc078_o](https://cloud.githubusercontent.com/assets/824194/4965468/b2a2d96c-6785-11e4-900f-52950ad24c87.jpg)