No liquidation: Vendors and MoC oracle changes

We've done some modifications which were asked by Money on Chain organization. These include:

get MoC token price using PriceProvider interface (peek)
changed the way a vendor can register (now anyone can do it) and unregister (only registered vendor can unregister)
a vendor will have to deposit a certain amount of MoC token when registering
added and modified tests accordingly

money-on-chain / main-RBTC-contract