Implement a/b/c... suffixes in references and citations, and create three-author fix

I've added some more tests, and I think that the resulting bbl files produce unambiguous citations when there are multiple papers by the same author in one year.

The three-author fix implements MN's stipulation that "For three authors, give all three names at first mention, e.g. (Brown, Jones & Smith 1994), but use first author et al. (in roman, not italic) thereafter, e.g. (Brown et al. 1994)."  However this requires a (noted) change to mn2e.cls, so it's currently disabled here (search for 'threeauthorhack').