Update Kerbanomics.cs

Added calculations for compound interest rates to the file...

FYI: my IDE for C-sharp is out of commission just now so I couldn't try compiling at home.  I'm almost positive c-sharp doesn't like the power notation(^) I used, but I left it in there so you could check it out.  I'll try to get my computer's ide working Monday when I'm back at work, but I'm stuck for now - got excited about your mod and wanted to help with the compound interest since I was calculated these things very recently due to a personal home-improvement loan I'm getting.
Cheers,
Mike