JSON Schema validation

### What is the problem / feature ?

The library has no mechanism for validating an api response against a declared swagger schema.

#### TODO

- [x] `multipleOf`
- [x] `maximum`
- [x] `exclusiveMaximum`
- [x] `minimum`
- [x] `exclusiveMinimum`
- [x] `maxLength`
- [x] `minLength`
- [x] `pattern`
- [x] `items`
- [x] `allOf`
- [x] `maxItems`
- [x] `minItems`
- [x] `uniqueItems`
- [x] `enum`
- [x] `maxItems`
- [x] `minItems`
- [x] `uniqueItems`
- [x] `enum`
- [x] `pattern`
- [x] `format`
- [x] `maxProperties`
- [x] `minProperties`
- [x] `required`
- [x] `readOnly`??
- [x] `externalDocs`

### How did it get fixed / implemented ?

`PropertySerializer.save()` now returns a callable that can be used to validate a json response against its declared schema.

### How can someone test / see it ?

Not sure yet what the api is going to be and how this fits in with the overall swagger schema.


*Here is a cute animal picture for your troubles...*

![enhanced-19397-1394043219-2](https://cloud.githubusercontent.com/assets/824194/4712466/a3bf92a2-58ce-11e4-845b-f7361a4dd35e.jpg)