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.

* Depending on the request Gitlab has a 'message' or 'error' attribute
in the json data, handle both
* Add some consistency by converting messages to unicode or str for
exceptions (depending on the python version)

Closes #616

POST and PUT requests are modified by clients when redirections happen.
A common problem with python-gitlab is a misconfiguration of the server
URL: the http to https redirection breaks some requests.

With this change python-gitlab should detect problematic redirections,
and raise a proper exception instead of failing with a cryptic error.

Closes #565

Closes #553