stop, retry: add compatibility wrappers for __call__ methods

[immerrr]

They can be called side-stepping the "primary" backward compatibility shim
provided by BaseRetrying class, for example when an inherited class uses an old
signature in a "return super().call"

This should fix #139

[immerrr]

Newly generated warnings look like this:

$ pytest -Wonce 
===================================================================================== test session starts =====================================================================================
platform linux -- Python 3.6.4, pytest-3.6.1, py-1.5.3, pluggy-0.6.0
rootdir: /home/immerrr/src/tenacity, inifile:
collected 80 items                                                                                                                                                                            

tenacity/tests/test_asyncio.py ..                                                                                                                                                       [  2%]
tenacity/tests/test_tenacity.py ...........................................................................                                                                             [ 96%]
tenacity/tests/test_tornado.py ...                                                                                                                                                      [100%]

====================================================================================== warnings summary =======================================================================================
<<...some warnings snipped...>>

  /home/immerrr/src/tenacity/tenacity/tests/test_tenacity.py:127: DeprecationWarning: "tenacity.stop.stop_after_attempt.__call__" method must be called with single "retry_state" parameter
    attempt_number, seconds_since_start)

tenacity/tests/test_tenacity.py::TestWaitConditions::test_exponential
  /home/immerrr/src/tenacity/tenacity/tests/test_tenacity.py:191: DeprecationWarning: "tenacity.wait.wait_exponential.__call__" method must be called with single "retry_state" parameter
    self.assertEqual(r.wait(1, 0), 2)

<<...some warnings snipped...>>

tenacity/tests/test_tenacity.py::TestWaitConditions::test_fixed_sleep
  /home/immerrr/src/tenacity/tenacity/tests/test_tenacity.py:154: DeprecationWarning: "tenacity.wait.wait_fixed.__call__" method must be called with single "retry_state" parameter
    self.assertEqual(1, r.wait(12, 6546))

<<...some warnings snipped...>>

  /home/immerrr/src/tenacity/tenacity/tests/test_tenacity.py:991: DeprecationWarning: "tenacity.retry.retry_if_exception.__call__" method must be called with single "retry_state" parameter
    return super(MyRetry, self).__call__(attempt)

[jd]

Thanks a lot @immerrr!

[jd]

There are some pep8 errors though :)

[immerrr]

@jd fixed