deposit: duplicate validator with prod inspirehep

One issue, but it's not a problem of the validator itself, rather our import logic: on import, even if there are some validation errors, the id is imported.

depends on: https://github.com/inspirehep/invenio/pull/115
I remember having the same comment and recommending [`requests`](http://docs.python-requests.org/en/latest/user/quickstart/), I think it is much nicer.

On the other hand, it is true that this check will be done while we have the 2 systems in parallel and later converted in a local check, so it is your choice.