Tackle source look-up code

[johenglisch]

I've been putting this off for a while but I think it's time I tackle that source look-up code (as much as it scares me)…

Some observations:

• Apparently batch lookup using grambank sourcelookup glottolog sheet1 sheet2 sheet3 ... works most of the time except when it doesn't… Don't know why…
• I caught the Unresolved sources output 'lying' a few times…
• afaict you can't just reference a source using a bibkey – it has to be Author Year… (correct me if I'm wrong)
• The program does not understand what an Online Tuvan dictionary (tyvan.ru) is.
• Infuriatingly, it does unterstand what a Sakha online dictionary is.
• In general, I have trouble understanding the code… (<_<)'

But before that, are PRs #97 and #98 good for merge?

[xrotwang]

I can relate to this :) Starting with tests (see https://github.com/grambank/pygrambank/blob/master/tests/test_srctok.py and https://github.com/grambank/pygrambank/blob/master/tests/glottolog/references/bibtex/hh.bib) might be the best way to ease into this topic.