SignalBuy contract

[NIC619]

• Modified from LimitOrder contracts
• It checks takerToken/makerToken ratio and make sure the ratio provided by taker is better than or equal to order's specified ratio
• Since maker can receive more taker token if takerToken/makerToken ratio is better, the token type of the filled token amount record is changed from taker token to maker token.
  • added orderHashToMakerTokenFilledAmount to LibOrderStorage
• add tests to check
  • worse takerToken/makerToken ratio can not be filled and
  • better takerToken/makerToken ratio can be filled fully/multiple times

[charlesjhongc]

Overall no particular issues to me. I think the app can test on it first to see whether it fits the need or not.