arxiv: OAI2 API usage and status code addition

* Changes the arXiv import source to OAI2 url.  (addresses #1866)

* Adds status code to API request.

* Updates existing unit tests and adds new ones: unsupported
  versioning and malformed.

* It's able to get more info than before.

Signed-off-by: Pedro Gaudêncio <pedro.gaudencio@cern.ch>
Reviewed-by: Jiri Kuncar <jiri.kuncar@cern.ch>
search: layout fix

* Fixes the shrunk search bar.

Signed-off-by: Yoan Blanc <yoan.blanc@cern.ch>
docs: installation on Centos

* Adds how to setup invenio as a Centos 6.5 user.

* Changes travis so it builds the docs and then runs the tests.

Signed-off-by: Yoan Blanc <yoan.blanc@cern.ch>
admin: administration menu fix

* Registers all subitems to the 'main.admin' menu item. (closes #1822)

Signed-off-by: Eirini Psallida <eirini.psallida@cern.ch>