Added a new Bibdata member option 'citation_label'. This is the first step in preparing for the new generate_bibitem_label() function, for which I've added a placeholder method. Also changed the member option 'citation_order' to 'citation_sort' since this better reflects what the option means when we allow the citation sortkey and the reference label to be completely different.
Added the Bibdata class method generate_bibitem_label() to bibulous.py. With the new split between options 'citation_sort' and 'citation_label', a bunch of the BBL files in the testing quite had changes. I also made some cosmetic changes to bibulous_test.py.
Added test #7 to the testing suite. This turned up a bug in generate_bibitem_label() when the author name begins with a curly brace (such as when the name begins with a LaTeX-markup for a non-English character). The solution was easy: use purify_string() on each name prior to extracting the characters.