arxiv: response code addition

* Also avoids error if a response message is not mapped with an HTTP
  code.

Signed-off-by: Javier Martin Montull <javier.martin.montull@cern.ch>
`get_json` returns a dict, it's only called _json_ because it internally deals with JSON content, but as a user of the API, I shouldn't have to care. On the other hand, `get_response("1410...").content` returns some JSON. Take some time to generate [the documentation](http://invenio.readthedocs.org/en/latest/ext/arxiv.html) and look at it to see if it makes sense. If `get_response` and `get_json` shouldn't be called from the outside, hide them properly.