some functions from countimp version 1.0

see http://www.uni-bielefeld.de/soz/kds/software.html
- Poisson imputation
- Quasi-Poisson imputation
- NB imputation
- zero-inflated Poisson and NB imputation
all functions available as Bayesian regression and bootstrap regression
variants
Merge pull request #3 from kkleinke/count

some functions from countimp version 1.0