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.
No, no problem. I just don't want to create the impression that I'm trying to prevent copying. I'll add it if you guys think I should.