Add #remove_from_fallback and call where appropriate
Refactor Entry#to_xml by introducing Entry::BibTeXMLConverter
Fix #convert!
Add BibTeXML fallback

The fields are added directly to the entry.
BibTeXML would usually require an <bibtex:entry> tag followed by e.g. <bibtex:book>. <bibtex:book> would then contain the fields.
As <bibtex:entry> nor <bibtex:book> are proper predicates, I omitted them for now.