crossref: initial release of Flask extension

* Queries DOI record data from the database or CrossRef as JSON
  (closes #1906).

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

* Adds an instance wide search parameter prefix to perform the database
  search.

* NOTE: for the search to be performed in the database, the DOI field
  prefix for the corresponding instance config file should be added to
  invenio.cfg as DOI_SEARCH_PREFIX (e.g. DOI_SEARCH_PREFIX = u'doi:').

Signed-off-by: Pedro Gaudêncio <pedro.gaudencio@cern.ch>
Reviewed-by: Jiri Kuncar <jiri.kuncar@cern.ch>
crossref: tests addition

* Adds tests for DOI found and not found.

Signed-off-by: Pedro Gaudêncio <pedro.gaudencio@cern.ch>
Reviewed-by: Jiri Kuncar <jiri.kuncar@cern.ch>
crossref: docs entry addition

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