Make customisation of completion route simple

mirroring Spree::CheckoutController#update

Fixes #62
set InvoiceID to order number

Fixes #76
Add german translation

Fixes #79
Stop coffee_script and sass warnings
Pass cancel token back to checkout

Reasoning: as it is we lose the information that an order has been canceled. Better to keep it around so programmers can customise post Paypal failure cancellation experience.

Fixes #63

Conflicts:
	app/controllers/spree/paypal_controller.rb
 merged in latest from upstream
 new payment logging
 merged in upstream