Adding a repayment method class for determining the repayment order.

The repayment method is initially used for simple sorting for repayment, but it is planned to allow dynamic ordering of the repayments to take into account changes that are pertinent to such methods as those that use ratios.
Adding a string representation of the debt.
Adding a repayment method for the lowest/highest interest rate first.
Adding the repayment methods for the balance/rate and balance/payment ratios.
Merge branch 'master' into repayment-methods
Bringing back the repayment part to the snowball library.
Adding the intl package dependency.