Many changes - continuing to update Zsh/tmux/vim configs

* Powerline now installed and working - replace vim-airline
* Fix gist-vim plugin's dependencies
* Add tmux.vim syntax file for .tmux.conf editing
* Continue cleaning up vimrc file
  * turn off smartindent
  * Add an autocmd to start NERDTree window if Vim started with no files
  * remove settings for Airline
  * Add settings for gist-vim