Python 3 compatibility

Hi there,

First of all, thanks for writing this library :+1: 

I have re-written some code in order to make it compatible with Python 3 + Django. Some settings are changed, for instance the BTW and MOLLIE_MIN_AMOUNT. Maybe you can pull this upstream or create a separate branch so that Python 3 users can also use and benefit from this library.

Have a good day and thanks again for writing this library,
Wouter.