Add SIP2 fee payment, the 37/38 message/response pairs.

Add SIP2 fee payment, the 37/38 message/response pairs.

* Pay individual bill if a fee id is given by the SC.

* Pay multiple bills in batch so if one fails the whole transaction
  is aborted and a failure response is sent to the SC.

* Reject payments outright if the SC sends an overpayment.

* Add constants for bill not found and overpayment messages.

* Whole lotta logging goin' on (maybe too much).

Signed-off-by: Jason Stephenson <jstephenson@mvlc.org>
Signed-off-by: Bill Erickson <berick@esilibrary.com>