Implement non-recurring ELV payment.

I know there has already been a commit implementing ELV for Adyen, however there is one *big* problem with it: 
It's only usable for recurring payments, not for one time payments like with credit cards.
Adjusting existing logic was out of the question since this would have led to spaghetti code.

The attached pull request hopefully fixes this with minimal changing of existing logic.

What I did:

a) Add a new API method
b) Add a new, minimal payment service
c) Add an ELV module which encapsulates elv logic
d) Add a new template
e) Move elv logic out of the recurring payment service in one place
f) Add specs

Also, the existing validations for ELV were too strict, checking for bank_location and bank_name as well - all you need is holder name, bank_location_id and number.

I tested this pull request against the Adyen API and it worked as expected.

Please let me know what you think about this.