Add Adyen direct gateway.

I need some feedback about some parts of this PR but I'm putting it out there so it can get reviewed. @bslobodin 
Do you know how they handle currencies with nonstandard minor units? It'd be good to have a test as well once you confirm what values are supposed to be sent over the wire.

https://github.com/Shopify/active_merchant/blob/a49815053a6d6feafb1e70f068fa684947404d9b/lib/active_merchant/billing/gateway.rb#L196
https://github.com/Shopify/active_merchant/blob/a49815053a6d6feafb1e70f068fa684947404d9b/test/unit/gateways/stripe_test.rb#L102