Cleanup ignat99 feat

New PR from https://github.com/Ignat99/OpenBazaar/pull/1 directly into OpenBazaar:develop.
I am OK with fixing null prices but I don't think we should fix negative prices silently. Maybe some old lady wants to purge her house of old junk and will *pay* anyone that comes and takes it away. She could "sell" her junk and charge a negative price for them.

This is just a thought; I don't know if someone can actually charge a negative bitcoin amount, maybe @dionyziz can enlighten us on the matter. In any case, a negative price is most probably an error and should be rejected (loudly, please) by the UI, not the notary processor. We should protect our users against accidental errors. I've read many stories about how people made buckets of dollars botting the Diablo 3 Auction House, relying on price typos and other such mishaps (one such story made the HN recently but I can't find it).

TL, DR: We shouldn't mix [mechanism and policy](https://en.wikipedia.org/wiki/Separation_of_mechanism_and_policy). We should be strict on our policy and not let possible errors fall through silently, but I don't think this code is the right place. Maybe @drwasho has something to contribute.