Improve vim statusline, remove powerline

This makes the statusline more useful without a heavy (and currently
broken for me) plugin. Taken from Wincent:

  https://github.com/wincent/wincent/blob/master/.vim/plugin/statusline.vim
Wrap remove trailing whitespace autocmd in augroup

This prevents it from doubling up if the file is re-sourced. More
information at:

  http://learnvimscriptthehardway.stevelosh.com/chapters/14.html