Update amortization_function.R

1. Updated the function to read multiple loan information. 
2. Changed the date format to m/d/Y since the default date setting in excel is m/d/Y. The function can process the dates directly when importing data set from csv.
3. Tested the function on 1000 loans with 6 installments each. System time elapsed=84.83
4. I didn't find any similar tools that can compute amortization schedule for simple interest with uneven installment due dates. So the unit test will be validating against  the amortization schedule in the example.