Working Compound Interest

Compound interest calculations are now working.
In addition, several other small bugs were fixed.
Taking multiple loans is now handled as taking sufficient principal to
pay back the previous loans and/or refinance to a new number of
payments.
Loan repayment now handles non-integer payments and should correctly
handle closing a loan when appropriate.  it should prevent the use from
overpaying.

The calculations for "yearly" should be ok, but I didn't test it much.