Expand API for step2_exchange

In python 3, the code received from the browser will be binary instead of a string. Expand the API for oauth_flow.step2_exchange(code) to allow this value to be passed as is rather than decoding.

for example:
credentials = self.flow.step2_exchange(b'some random code')

test:
tox -e py34  -- tests.test_client:OAuth2WebServerFlowTest.test_exchange_success_binary_code

resolves:  #443, #446