JP - Adds free algebra for Evaluator Client API

[juanpedromoreno]

This PR brings a preliminary version of the free algebra to interact with the evaluator service.

In this way, this would be an example of use case:

val client = new EvaluatorClient("<evaluator-service-url>",  "<evaluator-auth-key>")

  val response: Free[EvaluatorOp, EvaluationResponse[EvalResponse]] =
    client.api.evaluates(
      dependencies = List(
        Dependency(
          groupId = "org.typelevel",
          artifactId = "cats_2.11",
          version = "0.6.1")),
      code = "{import cats._; Eval.now(42).value}")

  response.exec[Eval].value match {
    case Xor.Right(EvaluationResult(result, _, _)) ⇒
      println(result)
    case Xor.Left(ex) ⇒ throw ex
  }

Please, @dialelo @raulraja Could you review it? Thanks!

[ghost]

Note that MonadError[Id, Throwable] is not realy a MonadError since Id values are computed eagerly and exceptions cannot be converted to values with MonadError#attempt. The definition of Id (something like type Id[A] = A) also makes it interact weirdly during implicit resolution.

My suggestion would be to remove this instance.

[ghost]

Other than the comment LGTM!