Implemented Test Coverage for Follow On Credits

Created Test::Business::CyberSource::FollowOnCredit
  Implemented attempt_follow_on_credit

In t/04-follow_on_credit.t
  Added Tests for Use Cases for Visa, Amex, MasterCard, Discover:
    Refund a Sale
    Refund a Capture
    Refund an Auth
    Two Refunds on One Sale
    Refund More Than Collected