__getattr__ raises wrong excetion

[st31ny]

I noticed that ProtocolBase.__getattr__ raises a KeyError, although it should raise an AttributeError. (https://github.com/cwacek/python-jsonschema-objects/blob/master/python_jsonschema_objects/classbuilder.py#L241)

This causes hasattr() to fail if the property does not exist:

delattr(test, 'foo')
assert not hasattr(test, 'foo')
…
File "/usr/lib/python3/dist-packages/python_jsonschema_objects/classbuilder.py", line 241, in __getattr__  
  raise KeyError(name)
KeyError: 'foo'

`

When I change this to AttributeError the code behaves as expected.