added zero inflated poission and nbinom density functions

Tested to by fitting to simulated data. Also compared reports of densities of dzipois and dzinbinom with zip=0 to R's output for dpois and dnbinom, for both log=TRUE and FALSE.

I had to use * \details to make it so that only the first part of the description showed up in "R style probability distributions". An empty line didn't break them up.