Make changes to base line object class based on invoice line object.
@line_number reader -> @line_number accessor
@line_id -> .line_id reader method
Replace all references to @line_id with line_id (method call)
Update line object spec for new capabilities
Update subline object and spec
subline object is now indefinitely extensible via use of the .parent_line method
Update invoice line item to use new subline object class
Update invoice line to use base line object class
Update invoice current item to use base line object class
Remove invoice line object from lib/olefs/common
Remove invoice line object spec from spec/olefs/