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.
Every file has a copyright attribution, so the idea is not to prevent copying.

**Disclaimer: I'm not a lawyer, what follows is my personal interpretation and might be utter nonsense.**

For other, more restrictive open-source libraries (e.g., GPL), holding the copyright means that you can distribute the code not subject to the license restrictions. With our license, there are practically no restrictions and the problem is moot. It becomes more a matter of giving credit where credit is due. And maybe making some company lawyers happy (but don't tell them I said this).