Windows Support

[zBugi]

I tried to build and install it on windows. there is just one mistake in Resource.cpp which can be fixed through this otherwise the favicon and so on will be having a file path like "C:\dev\my-service\lib\oatpp-swagger\res/favicon-16x16.png" which cannot be load:

#ifdef _WIN32
  const char otapp_char_directory_separator = '\\';
  const String otapp_string_directory_separator("\\");
#else 
  const char otapp_char_directory_separator = '/';
  const String otapp_string_directory_separator = "/";
#endif  

Resources::Resources(const oatpp::String& resDir, bool streaming) {

  if(!resDir || resDir->size() == 0) {
    throw std::runtime_error("[oatpp::swagger::Resources::Resources()]: Invalid resDir path. Please specify full path to oatpp-swagger/res folder");
  }

  m_resDir = resDir;

  if(m_resDir->data()[m_resDir->size() - 1] != otapp_char_directory_separator) {
    m_resDir = m_resDir + otapp_string_directory_separator;
  }

  m_streaming = streaming;
}

I think it could be done even better with c++17 since there is a system path library included. But I haven't tried that yet.