Mollie API 1.3.6

• Added the MandateId property to the PaymentRequest and PaymentResponse classes.
• Locale is now a string instead of an Enum. This is because Mollie can't seem to decide on which locale's they support and the current enum values were too restrictive. For backwards compatibility, I converted the Locale enum to a static class with some preset values, such as nl, en etc.
• Now using ConfigureAwait(false) in my library, to avoid deadlocks in certain situations. Courtesy of markwemekamp.
• Added the SepaDirectDebit PaymentResponse class, that holds specific values for this payment type. Thank you nvanrooij!