Use List interface instead of ArrayList for return types and method parameters

[Nandi]

By using the List interface consumers of the API have more flexibility in how they interact with it.

Example when using this library with Kotlin:
With ArrayList

val postmarkClient = Postmark.getApiClient(postmarkToken)
val messages = listOf(constructTemplateMessage(templateId, receiver, model)
postmarkClient.deliverMessageWithTemplate(messages as ArrayList<TemplatedMessage>)

With List

val postmarkClient = Postmark.getApiClient(postmarkToken)
val messages = listOf(constructTemplateMessage(templateId, receiver, model)
postmarkClient.deliverMessageWithTemplate(messages)