0.34: ClearingHouse, RiskEngine, ContractSpecification and more

This major redesign includes:

• New RiskEngine trait with IsolatedMarginRiskEngine implementation
• Improved limit order margining mechanism
• ClearingHouse used to settle the transactions, acting as a central counterparty
• ContractSpecification: Contains a few more configurations for the market
• MarketState now contains the current market information
• Many new methods with old ones removed
• Removed Margin struct
• Restructures internals heavily

This version requires consumers to call new methods, but the general principals stay the same.
Happy trading!