Generate Random Cost Within Range

Cost values were hard coded as exact amounts (Apples 2.51, Grapefruit 0.99, etc.), making for predictable game play.

* WBItem contains two new properties: costMin and costMax, both doubles.
* On init: costMin, costMax set; cost generated and set.
* cost will remain same value during one session, but vary between sessions.
* costMin and costMax for each item hard coded in WBItemStore.m