Integrated PR for all open issues

[jcoliz]

This PR resolves all known bugs and adds new Excel functions introduced since the library was originally created.

• Add PDURATION function. Returns the number of periods required by an investment to reach a specified value. (Resolves Implement PDURATION #62)
• Add RRI function. Also used for CAGR. Returns an equivalent interest rate for the growth of an investment. (Resolves Implement CAGR/RRI #60)
• Improve XIRR function by reducing the precision required before an answer is returned. (Fixes Find Root operation does not follow Excel spec as stated in comment => common.fs #27)
• Improve ACCRINT function by allowing first interest date on the settlement date. (Fixes Behavior of AccrInt does not match Excel when firstInterest <= settlement #22)
• Add PriceAllowNegativeYield function, which operates like the PRICE function except that it allows negative yield inputs. This is experimental. We'd love feedback on how this works for folks. (Fixes Can't calculate price with a negative yield #13)