Support for Ephemeral Keys Missing

[amacks]

The API uses an Ephemeral Key to handle Apple Pay xacts. There's currently no support in the library for that call. It is not well documented in the API docs, but works as the other API Put calls, requiring a
customer object:

  curl https://api.stripe.com/v1/ephemeral_keys  -u sk_test_XXXXXXX: -d customer=cus_XXXXX -H "Stripe-Version: 2018-02-28"

[amacks]

I've submitted a PR against this with a possible implementation of the EphemeralKey class: #105
No promises, but it seems to work as expected