populate - arg throw_on_unknown_keyword

Hi,

I would like the `__init__` and `populate` methods [ref](https://github.com/beregond/jsonmodels/blob/master/jsonmodels/models.py#L32), to have the following signature:

```python
def __init__(self, throw_on_unknown_keyword = False, **kwargs):
# ...

def populate(self, throw_on_unknown_keyword = False, **values):
# ...
if structure_name in values:
field.__set__(self, values.pop(structure_name))
elif throw_on_unknown_keyword and structure_name != 'throw_on_unknown_keyword' :
throw # ...
for name, _, field in fields:
if name in values:
field.__set__(self, values.pop(name))
elif throw_on_unknown_keyword and name != 'throw_on_unknown_keyword':
throw # ...

# warning: I have not checked the details!
```

Reason: I wrap the Model inside a class:

```python

class A:
class ModelA(models.Base):
a = fields.DateTimeField(required=True)
b = fields.FloatField(required=True)
c = fields.FloatField(required=True)

def __init__(self, **kwargs):
self.model = self.ModelA(throw_on_unknown_keyword = True, **kwargs)

# #Instead of
#def __init__(self, a, b, c):
# self.model = self.ModelA(a = a, b = b, c = c)
```

With `throw_on_unknown_keyword = True`, I keep the constructor of A and the model synchronized, and get an exception if they are not synchronized.

Is that feasible?