BibTeX/RIS/CrossRef: Add DOIs to non-supported item types (zotero#1233)

BibTeX: also add additional test, udpated tests,  and add better
handling of additional identifiers from notes (which were in my local
bibtex either from an old version by Emiliano or from an attempt by me;
either way they look good).

* CrossRef:
Also, some minor touch-ups like cleaning out tabs and spaces from empty
lines, adding semicolons, removing deprecated code.