Added ability to add additional messages in the ProcessingException JSON response.

I wanted the ability to do my validations at the preprocessor level. Unfortunately the default behavior of the ProcessingException only included the ability to have a single message. I've extended the ProcessingException with an additional parameter `additional_messages=dict()` to pass through additional messages in the JSON body of the response.

```
validation_errors = {'errors': {'username': ['Already exists.'],
				                'email': ['Invalid email address.']}
		             'status': 422}

raise ProcessingException(description='Validation errors occurred.", code=422,
                          additional_messages=validation_errors)
```

Returns the following JSON response.

```
HTTP/1.0 422 UNPROCESSABLE ENTITY
Content-Type: application/json
Content-Length: 182
Server: Werkzeug/0.9.6 Python/3.4.2
Date: Thu, 16 Oct 2014 08:47:48 GMT

{
  "errors": {
    "email": [
      "Invalid email address."
    ],
    "username": [
      "Already exists."
    ]
  },
  "message": "Validation errors occurred.",
  "status": 422
}
```
Change `additional_messages` to `details`, it is more concise and closer matches the purpose of the parameter.

Also, don't provide a dictionary object as a default argument; instead provide `None`, then change the line below to `self.details = details or {}`.