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
Add entity_use_code to account
Merge pull request #147 from recurly/entity_use_code

Add entity_use_code to account
Adds BillingInfo#amazon_billing_agreement_id support

Signed-off-by: Christopher Rogers <chrissrogers@gmail.com>
Merge pull request #149 from recurly/amazon-payments

Adds BillingInfo#amazon_billing_agreement_id support
Bump 2.3.2

Signed-off-by: Christopher Rogers <chrissrogers@gmail.com>
adding the bulk parameter
Adding some clarity to the recurly initializer

Some people tend to be confused by the naming here so I think adding
these comments might be helpful. I also think we should remove the
recurly.js v2 parameter as it seems to be confusing people. If a
developer is generating this file then we should assume they are
starting a new project and thus want them to use recurly.js v3
Merge pull request #148 from recurly/bring_clarity_to_initializer

Adding some clarity to the recurly initializer
Merge pull request #150 from recurly/add_bulk_field_to_postpone

adding the bulk parameter
bump to 2.3.3
Merge pull request #152 from recurly/bump_2_3_3

Bump to 2.3.3
Add invoice previews
Merge pull request #153 from recurly/invoice_previews

Add invoice previews
bump 2.3.4
Merge pull request #154 from recurly/bump-2.3.4

bump 2.3.4