Fix moment calculations for negative c in PG(b, c)

Closes #7
Merge branch 'feature/fix-neg' of https://github.com/ariddell/BayesLogit
into ariddell-feature/fix-neg

Due to my git naivate, I changed the moment calculations on master, then
tried to merge this.

I fixed the conflict I think.

Conflicts:
	Code/C/PolyaGamma.cpp