RFC: Restored old, 30x faster version of digamma

Commit b19013db795af2d578090fd5c2c3462e43f5c422, "Add Julia translation of psifn from SLATEC with interfaces di-, tri- … " added a flexible polygamma function, but converting digamma into a thin wrapper around polygamma makes it 30-100x slower on my laptop.  This patch keeps the polygamma function, but keeps the old digamma implementation for that important special case.

I also added a few additional tests from Tom Minka's Lighspeed toolbox (http://research.microsoft.com/en-us/um/people/minka/software/lightspeed/)