added Guest Checkout, check for signature at notification and redirect, send signature, timestamp, and order id instead of secret in helper, update parameter names
Guest Checkout now always enabled

Letting merchant choose whether or not to enable GC by setting
:credential4 to "true" just didn't seem correct, so I just force GC to
be enabled always
standardize amount param
actually, no need
Add credit to dwolla integration files
remove header comments
centralize the signature verification method
rename credentials params
update tests to reflect the change in credential parameter names
remote comment headers, fix credential var names, fix sig verify fn
Make #unstore method interface more consistent

While it's true that a number of gateways don't use the options
parameter in their #unstore method, having that parameter there with a
default value allows consumers of the API to call the method
consistently regardless of the gateway.

Closes #721.
fixed signature calculation method

and improved signature verification testing
fixed notification test check
don't verify signature if checkout failure
tests now use real callback and redirect data

also renamed test objects for clarity
Merge remote-tracking branch 'upstream/master'

Conflicts:
	lib/active_merchant/billing/integrations/dwolla.rb
	lib/active_merchant/billing/integrations/dwolla/common.rb
	lib/active_merchant/billing/integrations/dwolla/helper.rb
	lib/active_merchant/billing/integrations/dwolla/notification.rb
	lib/active_merchant/billing/integrations/dwolla/return.rb
	test/unit/integrations/dwolla_module_test.rb
	test/unit/integrations/helpers/dwolla_helper_test.rb
	test/unit/integrations/notifications/dwolla_notification_test.rb
	test/unit/integrations/returns/dwolla_return_test.rb
Remove unneeded require.
Stripe: Include swipe_data parameter if provided.
Use track_data instead of swipe_data on the AM side.
Include a track_data attribute on the credit card class.