Use snake case for method names



I noticed that attribute accessors have camelCase but the Ruby community prefers snake_case:

* https://github.com/rubocop/ruby-style-guide#snake-case-for-symbols-methods-and-variables

```diff
- person.stockLimitation = 0
+ person.stock_limitation = 0

- catalog_item.addOffer(offer)
+ catalog_item.add_offer(offer)
```

