Add credits to user account

[elimisteve]

Accept POST with the following JSON schema:

{
  "$schema": "http://json-schema.org/draft-04/schema#",
  "id": "http://sbhx.co/currency/api/1/credit",
  "type": "object",
  "required": false,
  "properties": {
    "card_id": {
      "id": "http://sbhx.co/currency/api/1/credit/card_id#",
      "type": "string",
      "required": false
    },
    "amount_in_cents": {
      "id": "http://sbhx.co/currency/api/1/credit/amount_in_cents#",
      "type": "integer",
      "required": false
    }
  }
}