added: Option for number of retries for HcfMiddleware

[starrify]

Sometimes it's observed that requests in the upstream python-hubstorage package may fail, e.g:

    Traceback (most recent call last):
      File "/usr/lib/python2.7/dist-packages/twisted/internet/base.py", line 1169, in run
        self.mainLoop()
      File "/usr/lib/python2.7/dist-packages/twisted/internet/base.py", line 1178, in mainLoop
        self.runUntilCurrent()
      File "/usr/lib/python2.7/dist-packages/twisted/internet/base.py", line 800, in runUntilCurrent
        call.func(*call.args, **call.kw)
      File "/usr/lib/pymodules/python2.7/scrapy/utils/reactor.py", line 41, in __call__
        return self._func(*self._a, **self._kw)
    --- <exception caught here> ---
      File "/usr/lib/pymodules/python2.7/scrapy/core/engine.py", line 106, in _next_request
        request = next(slot.start_requests)
      File "/tmp/eggs-3lbV9T/__main__.egg/radius/middlewares/hcf.py", line 147, in process_start_requests

      File "/tmp/eggs-3lbV9T/__main__.egg/radius/middlewares/hcf.py", line 220, in _get_new_requests

      File "/usr/lib/python2.7/dist-packages/hubstorage/frontier.py", line 60, in read
        return self.apiget((frontier, 's', slot, 'q'), params=params)
      File "/usr/lib/python2.7/dist-packages/hubstorage/resourcetype.py", line 40, in apiget
        return self.apirequest(_path, method='GET', **kwargs)
      File "/usr/lib/python2.7/dist-packages/hubstorage/resourcetype.py", line 34, in apirequest
        return jldecode(self._iter_lines(_path, **kwargs))
      File "/usr/lib/python2.7/dist-packages/hubstorage/resourcetype.py", line 27, in _iter_lines
        r = self.client.session.request(**kwargs)
      File "/usr/lib/python2.7/dist-packages/requests/sessions.py", line 461, in request
        resp = self.send(prep, **send_kwargs)
      File "/usr/lib/python2.7/dist-packages/requests/sessions.py", line 610, in send
        r.content
      File "/usr/lib/python2.7/dist-packages/requests/models.py", line 730, in content
        self._content = bytes().join(self.iter_content(CONTENT_CHUNK_SIZE)) or bytes()
      File "/usr/lib/python2.7/dist-packages/requests/models.py", line 662, in generate
        raise ConnectionError(e)
    requests.exceptions.ConnectionError: HTTPConnectionPool(host='storage.scrapinghub.com', port=80): Read timed out.

Simply retry that job and things may be fine.

The HubstorageClient class is configurable with an optional argument max_retries. Thus I think it may be a good idea to add a new option HS_MAX_RETRIES

[redapple]

@starrify , could you re-open this PR against https://github.com/scrapy-plugins/scrapy-hcf ?
Thanks.