Extend Invoice resource

[iranl]

• Adds listAll() function to Invoices
• Adds results/messages to Invoices through BaseObject
• Adds VAT lines
• Adds UnitsPriceInc to Sales invoice lines (for Invoice types that include VAT)
• Adds customerName, debitCredit and invoiceAmount variables which are returned when using listAll()
• Changes looping through Sales invoice lines from:

$responseDOM->getElementsByTagName('line')

To:

$linesDOM->childNodes

• Some formatting/order changes

[willemstuursma]

Thanks. I would like to use a bit more typing where appropriate. Would it be possible to add some new test cases?

[iranl]

Thanks. I would like to use a bit more typing where appropriate. Would it be possible to add some new test cases?

Thanks for the review and requested changes, will get on them in the coming week.

[iranl]

Superseded by #142