Add struct and API for books

[deepbrook]

We currently only provide the basic structs to build an HFT book. The following functions should be added to ease usage of this lib:

• Book struct and typedef, managing bids and asks, as well as limit and order_id maps.
• addOrderToBook Function
• removeOrderFromBook Function
• replaceOrderInBook Function
• Tests for these methods and structs in the CuTest test suite.