Implement a Router contract to add new functionality

[bghughes]

Add a Router contract that utilizes the low-level functions available in the protocol to provide new functionality and data. This router should implement the following functions:

• swap - the ability to route an order from one token on Rubicon to any other through the order book with some amount of slippage
• getBestOfferAndInfo: "It would be great to find the best offer AND retrieve its data in just one call. Today we have to first call getBestOffer which returns the orderId and with it, then perform a 2nd call to GetOrder(id) using the retrieved id. So, would be nice to change GetBestOffer to return the whole object instead of just the id"

Experimental functions to consider here:

• getOpenOrders(ERC20 sell, ERC20 buy): number of open orders for a given pair
• getOutstandingLimitOrders(address entity, address ERC20): outstanding orders in the order book for a given address.