return sampled activity status 'z' in all *.mcmc.DrawParameters methods
deprecated abe.mcmc.DrawFutureTransactions and abe.mcmc.PAlive, as their implementation is now identical with pcnbd.mcmc.DrawFutureTransactions and pcnbd.mcmc.PAlive
follow Abe's sampling via conjugate priors when drawing for Pareto/NBD
added reference to (Abe 2009)
* deprecated `abe.mcmc.DrawFutureTransactions` and `pcnbd.mcmc.DrawFutureTransactions`
 in favor of newly added `mcmc.DrawFutureTransactions` method
* deprecated `abe.mcmc.PAlive` and `pcnbd.mcmc.PAlive`
 in favor of newly added `mcmc.PAlive` method
bumped version number to 0.3.1 and added NEWS section