Support using "User-Interactive Authentication API" for login/register

Hi,

First of all, thanks for the SDK, that helps a lot!

Now onto my issue: I followed MatrixClient's example on registering an user and got back an error about captcha. Could this be a change in Matrix' protocol that wasn't propagated down here?

>>> from matrix_client.client import MatrixClient
>>> client = MatrixClient("https://matrix.org")
>>> token = client.register_with_password(username="foobar1234", password="password")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/dryvenn/Downloads/matrix-python-sdk/matrix_client/client.py", line 126, in register_with_password
    "m.login.password", user=username, password=password
  File "/Users/dryvenn/Downloads/matrix-python-sdk/matrix_client/api.py", line 110, in register
    return self._send("POST", "/register", content)
  File "/Users/dryvenn/Downloads/matrix-python-sdk/matrix_client/api.py", line 459, in _send
    code=response.status_code, content=response.text
matrix_client.errors.MatrixRequestError: 400: {"errcode":"M_UNKNOWN","error":"Captcha is required."}

Start with matrix_client/client.py at MatrixClient.register_with_password and follow the call into matrix_client/api.py's register and _send methods. Reproduce the shown registration request against matrix.org and inspect the User-Interactive Authentication API requirements. Done means login and registration support that API rather than failing with the captcha-required response.