message: curl_wrapper: add T to string method
  This is needed, because the c++11 equivalent is not working on Android.
diff: #include <iostream>
  #include <stdio.h>
  #include <fstream>
  +#include <string>
  namespace dronelink {
  @@ -88,6 +89,14 @@ size_t get_file_size(const std::string &path)
  return ((end - begin) > 0) ? (end - begin) : 0;
  }
  +template <typename T>
  +std::string to_string(T value)
  +{
  + std::ostringstream os;
  + os << value;
  + return os.str();
  +}
  +
  bool CurlWrapper::upload_file(const std::string &url, const std::string &path,
  const progress_callback_t &progress_callback)
  {
  @@ -110,7 +119,7 @@ bool CurlWrapper::upload_file(const std::string &url, const std::string &path,
  chunk = curl_slist_append(chunk, "Content-Encoding: ");
  // to allow efficient file upload, we need to add the file size to the header
  - std::string filesize_header = "File-Size: " + std::to_string(get_file_size(path));
  + std::string filesize_header = "File-Size: " + to_string(get_file_size(path));
  chunk = curl_slist_append(chunk, filesize_header.c_str());
  curl_formadd(&post, &last,
  