Add ServiceRequest API

[angelodipaolo]

As part of the work to separate the request API from ServiceTask, a new ServiceRequest struct type will be addded to replace for ServiceTask's requesting building API.

Currently ServiceTask API conflates request building with response processing:

service
  .POST("/brews")
  .setFormParameters(["foo": "bar"])
  .response { data, response in
    // process response
  }
  .resume()

With the new request API, ServiceRequest and ServiceTask values can be created independently for greater flexibility.

// create a request
var request = service.request(.post, "/brews")
request.formParameters = ["foo": "bar"]

// spawn a task instance for sending the request and processing the response
service
  .task(request: request)
  .response { data, response in
    // process response
  }
  .resume()