line_of_credit_test

Create an implementation of the following:

A line of credit product.  This is like a credit card except theres no card.
It should work like this:

  - Have a built in APR and credit limit
  - Be able to draw ( take out money ) and make payments.
  - Keep track of principal balance and interest on the line of credit
  - APR Calculation based on the outstanding principal balance over real number of days.
  - Interest is not compounded, so it is only charged on outstanding principal.
  - Keep track of transactions such as payments and draws on the line and when
    they occured.
  - 30 day payment periods.  Basically what this means is that interest will not be
    charged until the closing of a 30 day payment period.  However, when it is charged,
    it should still be based on the principal balance over actual number of days outstanding
    during the period, not just ending principal balance.

Couple of Scenarios how it would play out:

Scenario 1:

Someone creates a line of credit for 1000$ and 35% APR.

He draws 500$ on day one so his remaining credit limit is 500$ and his balance is 500$.  
He keeps the money drawn for 30 days.  He should owe 500$ * 0.35 / 365 * 30 = 14.38$ worth
of interest on day 30.  Total payoff amount would be 514.38$

Scenario 2:

Someone creates a line of credit for 1000$ and 35% APR.

He draws 500$ on day one so his remaining credit limit is 500$ and his balance is 500$.
He pays back 200$ on day 15 and then draws another 100$ on day 25.  His total owed interest on
day 30 should be 500 * 0.35 / 365 * 15 + 300 * 0.35 / 365 * 10 + 400 * 0.35 / 365 * 5  which is
11.99.  Total payment should be 411.99.