Added billing address to creditCard interface

Signed-off-by: Tom Wright <tom@inflatablecookie.com>
Added currency abstraction

Signed-off-by: Tom Wright <tom@inflatablecookie.com>
Stubbed mint gateways

Signed-off-by: Tom Wright <tom@inflatablecookie.com>
Stubbed stripe mediator

Signed-off-by: Tom Wright <tom@inflatablecookie.com>
Fixed url base to allow empty password strings

Signed-off-by: Tom Wright <tom@inflatablecookie.com>