Support Discount Line Items

[akinsey]

Add support for new discounts property of invoice object.

var invoice = {
  //...
  discounts: [{
    desc: "Military Discount",
    amount: 10.5,
  },
  {
    desc: "Other Discount",
    percentage: 25
  }]
  //...
};

[akinsey]

Support added in commit d524db2

Still need to add mocha test

EDIT: Support was supposed to be added in the commit mentioned above, but I messed up the commit.

This issue is closed in 0e610bc