set InvoiceID to order number

Fixes #76
Paypal Refund
Merge branch 'refund' of github.com:DangerDawson/better_spree_paypal_express into 2.1

# By jerome
# Via jerome
* 'refund' of github.com:DangerDawson/better_spree_paypal_express:
  Paypal Refund

Conflicts:
	app/models/spree/gateway/pay_pal_express.rb
	spec/models/pay_pal_express_spec.rb
Merge branch '2-1-stable' of github.com:radar/better_spree_paypal_express into 2.1

* '2-1-stable' of github.com:radar/better_spree_paypal_express:
  set InvoiceID to order number