refactored tmux.conf to isolate status bar settings
Added tmux-powerline to dotfiles.

Using tmux-powerline to create tmux status bar.

Cloned https://github.com/erikw/tmux-powerline to .dotfiles.
Created my own theme, generated the $HOME/.tmux-powerlinerc file.
Moved $HOME/.tmux-powerlinerc to .dotfiles and added a symlink.
Updated README.
Move my tmux-powerline theme to root of .dotfiles.

Changed TMUX_POWERLINE_DIR_USER_THEMES to .dotfiles so that
mytheme.sh can be kept there, versioned there, and not muck up the Git
status of the tmux-powerline repository.