Add STCPay and ApplePay sources

Add applepay and stcpay sources (#13)

* Support ApplePay and STCPay

* update packages

* update version

v0.6.1: Install requirements update + __str__ returns obj as json (#7)

* renamed constructor.py to helpers.py to add new class for __str__

* Implemented __str__ for Resource and Source

v0.6.0: Setup.py modifications (#3)

* setup.py, minor fixes to create() and refund() and package security update.

* passing only source to source builder instead of the whole payment object

* setup.py mods