Fixed error in Payback Period formula

[B-D-T]

Fix for finance.PP

Summary of the problem

The payback period calculation does not return the correct result. It includes and extra year, then subtracts the fractional final year instead of adding it.

Demonstration of the problem

The example, finance.PP(5, -50, 10, 13, 16, 19, 22);, says the pp is 3.42, but it should be 3.57895 (3 full years and .579 of the fourth year). We can see that in the cumulative return:

   0    1    2    3    4    <-- Years
-- | -- | -- | -- | -- | --
 -50   10   13   16   19    <-- Cash flows 
 -50  -40  -27  -11    8    <-- Cumulative return

After year 3, there is still $11 to recover. We'll get $19 in year 4, so we'll break even 11/19th of the way through year 4
(11/19 = 0.57895).

Solution

Small change in line 83 in finance.js:

Current: yearsCounter += (cumulativeCashFlow - arguments[i]) / arguments[i];
Fixed: yearsCounter += -1 + Math.abs((cumulativeCashFlow - arguments[i]) / arguments[i]);

[B-D-T]

This is connected to issue #51 I apologize for creating two pull requests (new to GH).