Adding the basic amortization class.

The amortization class is used for keeping track of a series of payments to a debt that is tracked decoupled from the debt.
Adding the amortization tests to all_test and fixing the package name.
Adding a varied payment test.
Adding tests for the payoff amount.
Adding tests for the payment class.
Switching to use single library and parts.
Removing reference to non-existant repayment part.
Merge branch 'master' into amortization-basic-tests