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.
Yes, I'm a bit unsure about how copyrights work. What I did was base my code on the existing lookback option code, so I didn't want to remove those copyright attributions. I added the new code in my own capacity, not in my company's capacity. Please advise how I should attribute.