missing socket.io-client-cpp

```
CMake Error at CMakeLists.txt:36 (add_subdirectory):
  add_subdirectory given source "src/engine/external/socket.io-client-cpp"
  which is not an existing directory.
```

I suppose I could lookup that library my self and copy paste it in there but thats nuts. The version might be wrong. This should be included in the repo. You could add it as a git submodule or have cmake pull it.