no deepcopy

"Bug": copy.deepcopy is not possible.

```python
from jsonmodels import models, fields, validators
import copy

class A(models.Base):
x = fields.IntField(required=True)

a1 = A(x = 1)
a2 = copy.deepcopy(a1)
print(a1.x)
print(a2.x) # fails in this line
```