Added mean, mode & variance functions to gamma distribution
Added mean, mode & variance functions to gamma distribution

ggonzale/jstat,jamescgibson/jstat,NorthDecoder/jstat,cmpolis/jstat,jstat/jstat,akrawitz/jstat,JeebsM/jstat,smarden1/jstat,NorthDecoder/jstat,jstat/jstat,JeebsM/jstat,akrawitz/jstat,ggonzale/jstat,smarden1/jstat