Added match-once to search the order book and match order according to
priority.
Added test corresponding to match-once
Added HTTP backend server
Tests for backend server
Market depth was not actually giving the market depth, but the order
count. Created a new count-orders with the previous code and created a
new market-depth function that actually adds up the sizes of all orders.

Also, added a clear-book function to reset the book (useful for
testing).
Updated main program.
Updated tests for the engine.
Orders are sent to the HTTP server and we check that the order book
actually matches the orders. Also, a test sends orders in bulk with
random sizes and sides, checking that orders sent are matched between
themselves and that either the bid or ask (the one with larger number of
orders -size-) remains with outstanding orders.