Expose a browser facing API for charge

Created by: vu3rdd

First cut towards #24 (closed). The webhook API is retained as it is.

Tested (with emacs restclient-mode) with the following:

POST http://localhost:8081/v1/stripe/charge
Content-Type: application/json

{ "token":"tok_visa", "voucher":"abcdefg", "amount":999, "currency":"USD" }