TEST: Fixed creation and deletion tests.
TEST: Import a record from ACM.
FIX: Import a record from ACM.

* In BibTeX records, `url` is a synonym for `ee`.
* Authors are separated with a comma and a space.
TEST: Import a record from DBLP.

Without proceedings data for now.
TEST: Refactoring for more explicit failure messages.
TEST: Import a record from SCOPUS.
FIX: On imports, use DOI HTTP-URIs whenever it is possible.