Use Paymill instead of Stripe

I would really like to use this gem for my application but i am based in germany, so i cannot use Stripe. But we have a perfect clone of stripe, called paymill (https://www.paymill.com/de-de/). In this branch i will try to use paymill instead of stripe and hopefully it can be merged in your master and decides intelligent and transparent for the user which library to take.

I am not ready at all, just started this pull request to let you know about my plans so that you can tell me what you think about it, hope you'll like it.