Update Changelog
Create invoice elements
Update version information (v2.6.5 -> v2.6.6)
Update wording of compatibility constant
Add invoice xpath elements
Create invoice line spec
Create Invoice PO Line
Update version (v2.6.6 -> 2.7.0) for new functions

Fixing version to be consistent with semantic versioning
Implement invoice line item
Shift Invoice Classes from OLELS to OLEFS
Create OLEFS Invoice Line Object base class for DRYness
Add prior total field to invoice

(For total prior to additional charges)
Add @ole.quit to invoice line specs
Create invoice current item line object
Fix describe string in OLELS Return page spec
Merge branch 'development'