[RFC] Use UIDs as remote object handles returned by the API

This was [commented](https://github.com/neovim/neovim/pull/582#issuecomment-42772184) in #582: Every buffer, window and tabpage has an unique id for the lifetime of nvim, which ensures "correctness" for remote objects manipulated by clients.

For testing with python-client use the latest master(not the pypi version)