Partial lookbacks

As discussed in http://quantlib.10058.n7.nabble.com/Lookback-options-with-partial-floating-strike-td15662.html

This implements partial-time fixed and floating strike lookback options using the formulas developed by E.G. Haug in Option Pricing Formulas, Second Edition. Test cases from literature.
If you did it on your own time, it's probably your copyright. But to remove all uncertainty, I'd ask your boss.