Fix CreditCard check
User agent in base gateway class.

Conflicts:
	lib/active_merchant/billing/gateway.rb
Fix app55 user agent.
Merge pull request #1181 from Shopify/user_agent

Common user agent in base gateway class.
Generic normalize method.

Conflicts:
	lib/active_merchant/billing/gateway.rb
Merge pull request #1182 from Shopify/normalize

Generic normalize method.
Put a generic expdate method in the Gateway class to reduce duplication.

Conflicts:
	lib/active_merchant/billing/gateway.rb
Move expdate to CreditCardFormatting.
Merge pull request #1180 from Shopify/fix_nmi

Fix CreditCard check for NMI
Merge pull request #1183 from Shopify/expdate

Put a generic expdate method in the Gateway class to reduce duplication.