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
Paypal Express gateway: Remember billing agreement

The Paypal Express #store method uses Paypal's CreateBillingAgreement
API call.  The response to this call gives us the billing_agreement_id
which can then be used for future charges via the DoReferenceTransaction
API call.

This commit allows us to consistently ask the response for its
authorization.

https://www.x.com/developers/paypal/documentation-tools/api/createbillingagreement-api-operation-nvp

Closes #656.
Paypal Express gateway: Allow an amount of 0

No longer default the amount to a dollar.  There are times when an
amount of 0 is perfectly fine for a Paypal Express transaction.

In fact, an amount of 0 is required if you'd like to do an authorization
without an immediate charge.

https://www.x.com/developers/paypal/documentation-tools/express-checkout/integration-guide/ECReferenceTxns

Closes #655.
Paypal Express gateway: Reduce param requirements

For reference transactions, Paypal Express does not require a payment
type, an invoice_id, a description, or an IP.  We shouldn't require them
either.

They can certainly be specified and we pass them to Paypal
appropriately, but they're not required.

Closes #657.
Quickpay integration: Update to handle API v6

Fixed that the quickpay notification parser failed to parse v6
notifications both because they are now URL-form encoded (since v4), and
because it was missing two fields in the checksum calculation

Closes #658.
Add 2.0.0 support at travis-ci, and allow failure
Merge pull request #663 from sanemat/feature/travis-ruby-2-0

Add 2.0.0 support at travis-ci, and allow failure
Sage gateway: Deprecate #credit call

Use #refund instead.

Closes #666.
Update notification generator

Notification template's parse method is a bit outdated. This is to make
it in sync with latest notification class. I also documented that fact
that notification initialization requires raw post string.

Closes #664.
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