fix situation leading to infinite derivative, add test case
Add more pay-off types to payoffTypeToString
Merge pull request #116.
Merge pull request #119.
empowered implied vol methods for displaced diffusion
better handling of evaluation dates which are not business dates
Give names to intermediate results.

Bytes on the stack are cheap these days, so it's not necessary
to reuse data members at the expense of clarity.
Merge pull request 124.