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
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
Merge pull request #722 from Dwolla/master

Dwolla Integration major bug fix & improved tests