func findContactId(withEmail email: String) -> [Int] {
        let resource = Resource<[Int]>.init(method: .GET,
                                     path: ApiPath.contactsFilterByEmail + email,
                                     json: nil) { (result) -> [Int] in
                                        let json = (try! JSONSerialization.jsonObject(with: result, options: [])) as? [String: [Int]]
                                        return (json?["ContactIdentifiers"]! }

        return performRequest(resource)!
    }

    private struct Resource<T> {
        let method: HttpMethod
        let path: String
        let json: Data?
        let parser: (Data) -> T
    }

    enum HttpMethod {
        case GET
        case POST
        case PUT
    }

    private func performRequest<T>(resource: Resource<T>) -> T {
        let url = URL(string: "\(apiHost)/\(account.id)/\(path)")!
        var request = URLRequest(url: url)

        if json != nil {
            request.httpBody = json
            request.setValue("application/json", forHTTPHeaderField: "Content-Type")
        }

        request.setValue("Bearer \(getToken())", forHTTPHeaderField: "Authorization")
        request.httpMethod = "\(method)"

        var result: Data?
        let semaphore = DispatchSemaphore(value: 0)
        URLSession.shared.dataTask(with: request) { data, response, error in
            guard let data = data, error == nil else {
                self.fail(withMessage: error?.localizedDescription ?? "No error data")
                return
            }

            if let httpStatus = response as? HTTPURLResponse, httpStatus.statusCode != 200 {
                self.fail(withMessage: "StatusCode should be 200, but is \(httpStatus.statusCode)")
            }

            result = data
            semaphore.signal()
        }.resume()

        semaphore.wait()
        return result
    }