Provide another way to send data to gitlab with a new `query_parameters`
argument. This parameter can be used to explicitly define the dict of
items to send to the server, so that **kwargs are only used to specify
python-gitlab specific parameters.

Closes #566
Closes #629

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.

http to https redirection cause problems. Make notes of this in the
docs.

Closes #579

Drop the code, the tests, and update the documentation.