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
Swap out REXML for Nokogiri in payflow common parser.

REXML is very strict with its xml parsing so fails hard on invalid
characters, Nokogiri strips them and continues on. This is a much better
experience for ActiveMerchant users.

I've only replaced the parser in Payflow common, but we should work on
moving everything to Nokogiri since it is a much better XML parser.
Merge pull request #673 from jduff/use_nokogiri_parser_with_paypal

Swap out REXML for Nokogiri in payflow common parser.
Transliterate the description for Payflow Link
Remove non-ASCII characters using gsub when transliterate isn't available
Fix Payflow Link tests for 1.8
Merge pull request #674 from Shopify/payflow_link_transliterate

Transliterate the description for Payflow Link
Paymill: Change .com -> .de

This fixes an issue where card tokenization was failing in production.

Also uses the correct marketing form - PAYMILL - and adds some supported
countries.

Closes #672.
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
Quickpay integration: Fix v6 response parsing

The regexp didn't work with the actual input directly from Quickpay.

Closes #661.
First Data e4: Add store support via TransArmor

TransArmor is an API extension for implementing card number
tokenization.

Closes #563.
MerchantWarrior: Format expiration correctly

According to the MerchantWarrior API documentation, cardExpiryYear and
cardExpiryMonth must meet these conditions:

  cardExpiryMonth
  Example: 05
  Notes: This must be in MM format. The month must be zero-padded if
         it’s less than 10.
  Valid Length: 2 digits.

  cardExpiryYear
  Example: 13
  Notes: This must be in YY format.
  Valid Length: 2 digits.

Closes #665.