Added JSON Marshaler for mollie's Shortdate formater

Implementation: #56

2020 Q1 housekeeping

Some minor changes were adapted to the library as described on mollie's developer changelog.

Subscription ID available on Payments resource

#54 adds SubscriptionID as optional parameter to the Payments struct, useful when getting webhook calls from Mollie.

This is a non breaking change and as an patches v1.2.0

Added Mandates API Resource

Based on the report in #51

Mandates API v2 implementation
Get mandate
List mandates
Revoke mandate
Create a mandate

Added missing optional field to the Subscription struct
Mandate ID

API parity release #49

Integrated changes for Payments, Orders and PaymentDetails reported in the API changleog during December 2019

bugfix #47

Fixes the error reported on issue #47

Initial release

Merge pull request #46 from VictorAvelar/housekeeping-11.2019

Housekeeping 11.2019