Merge pull request #52 from WriteOrRead/refactor

updating elements
Added basic fields to the billing request entity
* Added tracking of consumption data (eg. bandwith consumed)
* Added billing period start & end
* Adjusted billing agreement fields
* Added fiscal year & invoice type to Invoice
* Added an InvoiceItem interface
Merge pull request #53 from inspiran/refactor

* Adjusted billing agreement fields
remove BasePartner, update Partner to match interface