WIP ext: CrossRef Flask extension

* Queries DOI record data from the database or CrossRef as JSON.

* Closes #1906.
Actually, I just realised that I have to do it in a different way...Before I was [using  CrossRef openurl](https://github.com/pedrogaudencio/invenio/commit/f58a3620bc179c73fb5be903ab7ac9d22cce9143) instead of the REST API and I was getting the content in XML. Now it's already JSON, so I was thinknig of changing [`get_marcxml_for_doi()`](https://github.com/inveniosoftware/invenio/blob/pu/invenio/utils/crossref.py#L53) to receive JSON, convert it into XML and finally into MarcXML. What do you think, @greut?