WebSubmit: doilookup function in webinterface

* Adds doilookup function to websubmit_webinterface that uses the
  crossrefutils and returns a dictionary with MARC codes as keys and
  values from crossref website as values (that can be used in
  submissions to prefill some fields based on the DOI).

Signed-off-by: Sebastian Witowski <sebastian.witowski@cern.ch>
`defaultdict` is part of the batteries since python [2.5](https://docs.python.org/2.7/library/collections.html?#collections.defaultdict), is it still important to care?