Add separate line item and open amount refund methods to Invoice class

Example calls to method would be:

For Open Amount Refunds:
invoice.refund_amount(100)

For Line Item
charge = invoice.line_items.first
invoice.refund([{uuid: charge.uuid, quantity: 1, prorate: false}])

(D08-C) Line Item API to Allow Open Amount - Ruby
https://www.pivotaltracker.com/s/projects/975858/stories/71441512
Send charge in line items array instead of uuid and build it in xml

from the charge passed in instead.
Merge pull request #13 from recurly/refresh

Refresh with public
Specs and fixtures for refund methods on Invoice and field name corrections
Merge pull request #12 from recurly/jh-line_item_refunds

Add separate line item and open amount refund methods to Invoice class
Merge branch 'line_item_refunds'