Adapt neovim_mod.py to the lastest version of the 'neovim' package

The 'neovim' package was refactored and some parts of the API changed. This
updates neovim_mod.py to reflect those changes.
Merge pull request #57 from tarruda/fixes-for-latest-neovim

Adapt neovim_mod.py to the lastest version of the 'neovim' package