signable_bytes() and to_message_json() throw exceptions

Code in question:

```py
from eip712_structs import EIP712Struct, Address, String, Uint

class Identity(EIP712Struct):
userId = Uint(256)
wallet = Address()

class Message(EIP712Struct):
actionType = String()
timestamp = Uint(256)
authorizer = Identity

class EIP712Domain(EIP712Struct):
name = String()
version = String()
chainId = Uint(256)
verifyingContract = Address()

verifying_contract_domain = EIP712Domain(
name='VerifierApp101',
version='1',
chainId=8995,
verifyingContract='0x8c1eD7e19abAa9f23c476dA86Dc1577F1Ef401f5'
)

user_identification = Identity(userId=123, wallet='0xCcCCccccCCCCcCCCCCCcCcCccCcCCCcCcccccccC')
msg = Message(actionType='Action7440', timestamp=1570112162, authorizer=Identity)

print(msg.to_message_json(verifying_contract_domain))
print(msg.signable_bytes(verifying_contract_domain))
```

`msg.to_message_json(verifying_contract_domain)` throws the following exception:

```
Traceback (most recent call last):
File "submit_proof.py", line 32, in
print(msg.to_message_json(verifying_contract_domain))
File "/Users/anomit/.pyenv/versions/ev-cli/lib/python3.6/site-packages/eip712_structs/struct.py", line 188, in to_message_json
return json.dumps(message, cls=BytesJSONEncoder)
File "/Users/anomit/.pyenv/versions/3.6.5/lib/python3.6/json/__init__.py", line 238, in dumps
**kw).encode(obj)
File "/Users/anomit/.pyenv/versions/3.6.5/lib/python3.6/json/encoder.py", line 199, in encode
chunks = self.iterencode(o, _one_shot=True)
File "/Users/anomit/.pyenv/versions/3.6.5/lib/python3.6/json/encoder.py", line 257, in iterencode
return _iterencode(o, 0)
File "/Users/anomit/.pyenv/versions/ev-cli/lib/python3.6/site-packages/eip712_structs/types.py", line 244, in default
return super(BytesJSONEncoder, self).default(o)
File "/Users/anomit/.pyenv/versions/3.6.5/lib/python3.6/json/encoder.py", line 180, in default
o.__class__.__name__)
TypeError: Object of type 'OrderedAttributesMeta' is not JSON serializable
```

`msg.signable_bytes(verifying_contract_domain)` throws:

```
Traceback (most recent call last):
File "submit_proof.py", line 32, in
print(msg.signable_bytes(verifying_contract_domain))
File "/Users/anomit/.pyenv/versions/ev-cli/lib/python3.6/site-packages/eip712_structs/struct.py", line 200, in signable_bytes
result = b'\x19\x01' + domain.hash_struct() + self.hash_struct()
File "/Users/anomit/.pyenv/versions/ev-cli/lib/python3.6/site-packages/eip712_structs/struct.py", line 132, in hash_struct
return keccak(b''.join([self.type_hash(), self.encode_value()]))
File "/Users/anomit/.pyenv/versions/ev-cli/lib/python3.6/site-packages/eip712_structs/struct.py", line 61, in encode_value
encoded_values.append(sub_struct.hash_struct())
TypeError: hash_struct() missing 1 required positional argument: 'self'
```

Reproduce the example and trace struct.py through to_message_json(), signable_bytes(), hash_struct(), and encode_value(); inspect types.py for the JSON encoder involved in the first traceback. Compare the nested authorizer value with the defined user_identification instance, then establish the intended behavior and add coverage for both methods before confirming the exceptions are resolved.

Written by the indexing model from the issue text.