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.
Also, don't bother making another commit. I'll add the attribution when I merge the PR.