Revenue for a Merchant

[nicorithner]

This endpoint should return the total revenue for a single merchant.

The URI should follow this pattern: GET /api/v1/merchants/:id/revenue

Example JSON response for GET /api/v1/merchants/1/revenue

{
  "data": {
    "id": null,
    "attributes": {
      "revenue"  : 43201227.8000003
    }
  }
}