Fix missing "Retry-After" header and fix integration tests

The integration tests failed because a test called 'decode()' on a
string-type variable - the GitLabException class handles byte-to-string
conversion already in its __init__. This commit removes the call to
'decode()' in the test.

```
Traceback (most recent call last):
  File "./tools/python_test_v4.py", line 801, in <module>
    assert 'Retry later' in error_message.decode()
AttributeError: 'str' object has no attribute 'decode'
```

When requests are throttled (HTTP response code 429), python-gitlab
assumed that 'Retry-After' existed in the response headers. This is
not always the case and so the request fails due to a KeyError. The
change in this commit adds a rudimentary exponential backoff to the
'http_request' method, which defaults to 10 retries but can be set
to -1 to retry without bound.

fix(api): make reset_time_estimate() work again

Closes #672

docs(readme): provide commit message guidelines

Fixes #660

docs(setup): use proper readme on PyPI