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
}
```
If we're going to do it this way, I would like this method to return both the JSON-ified body and the status code. There's no reason to split up the return value (the JSON here and the status code in the calling function).

More briefly, change the return statement to read `return jsonify(message=message, **self.details), self.code`.