Added bindings for collection_method, net_terms and po_number elements on Subscription for manual invoicing. Added test around these new additions
setters for collectionMethod and poNumber take an object now instead of a string to work around some of Recurly's complex XML formatting
Merge remote-tracking branch 'upstream/master' into new_fields
Merge pull request #19 from patsimon/new_fields

Added bindings for collection_method, net_terms and po_number elements o...