A ValueError thrown by Model initialization returns a 400 Error.

This is how the ValueError goes down:

>>> from schematics.models import Model
>>> from schematics.types import IntType
>>> class Schema(Model):
...     number = IntType()
...
>>> Schema(**{"number": 1})
<Schema: Schema object>
>>> Schema(**{"number": "one"})
Traceback (most recent call last):
  File "<console>", line 1, in <module>
  File "[...]/eggs/schematics-0.6-py2.7.egg/schematics/models.py", line 140, in __init__
    setattr(self, attr_name, field_value)
  File "[...]/eggs/schematics-0.6-py2.7.egg/schematics/types/base.py", line 423, in __set__
    value = self.number_class(value)
ValueError: invalid literal for int() with base 10: 'one'
provides decorator does not accept model parameter.
Raise HTTPError when not returning a model

In cases where something else than a model is returned from a request
method, a HTTP 500 error is now thrown and logged properly. Before this
was simply ignored and resulted in an empty response with code 200.

Thanks @tobigue

Fixes #5
Updating Changelog and Authors