Additional enhancements for ParetoNBDModel

[ColtAllen]

While working on #177, I found quite a few things in the literature that can be added to this model, but I'd rather do so in separate PRs:

• marginal_posterior_transaction_rate() from (9) in http://www.brucehardie.com/notes/015/additional_pareto_nbd_results.pdf
• marginal_posterior_churn_rate() from (12) in http://www.brucehardie.com/notes/015/additional_pareto_nbd_results.pdf
• expected_purchases_interval() from (8) in https://www.brucehardie.com/notes/013/Pareto_NBD_interval_pmf_rev.pdf
• expected_purchases_probability_new_customer() from (16) in https://www.brucehardie.com/notes/012/pareto_NBD_pmf_derivation_rev.pdf
• discounted_lifetime_value() from (2) in http://brucehardie.com/papers/rfm_clv_2005-02-16.pdf
• Time-invariant covariates: Add Time-Invariant Covariates to ParetoNBD and BG/NBD Models #134