Add support for URI parameters in generated Swift code

[epankratov]

Some of API specs could have parameters, encoded in the URI. And Helium is not able to produce correct output in the Swift which allows to use such parameters. For instance, for the request:

  get "/service_config" spec {
    name "Available services"
    description "Returns available services for a specified affiliate."
    parameters {
      where 'string'
      max_results 'int32'
    }
    response "AffiliateServiceListResponse"
    httpHeaders header('Cache-Control', 'no-cache')
  }

it produces output like this:

    public func getServiceGroup(where: String, maxResults: Int) -> SignalProducer<AffiliateServiceGroupListResponse, APIResponseError<NSError>> {
        let url = URL(string:baseURL.absoluteString + "/service_group")!

In this output parameters aren't applicable at all, it must be something like this:

        let url = URL(string:baseURL.absoluteString + "/service_group?where=" + whereClause + "&max_results=" + String(maxResults))!

[roman-mazur]

Looking at the current code, we should collect URL query parameters in a separate collection (with URL encoded values), expose it in the mustache context and change the template to add these parameters.