ENH: Update on value function iteration.

Big update here.  Cramming a lot of stuff in one since this is the first
time its in a commitable state.  bellman can be called in two ways for
value fucntion vs. policy rule.
Merge branch 'master' of https://github.com/TomAugspurger/dnwr-zlb

Conflicts:
	model/value_function.py