Making minimum reward cost == 0.

In response to #47. Sets the minimum cost to 0. In the absence of Racer validation hooks (they say they're working on them but haven't updated that branch in over a year), I just used an 'on set' event, checked the value, then re-set it if it was below 0. Perhaps inelegant, but effective enough.

Ideally, a Connect middleware for validating against a model schema of some kind would be much better.