introduce price level to orderbook
remove order from price level
remove order from orderbook
rename new orderbook to limit orderbook, recover old orderbook
refactor limit orderbook
limit orderbook top
add LimitOrderBook#fill_top
add Order#crossed?
new matching engine
remove old engine and orderbook
fix engine dump in matching worker
fix order cancel
Merge pull request #96 from peatio/new-engine-with-position

New engine with position