Add buffer var to disable displaying trailing spaces

When using Unite's `unite_settings` the show setting didn't work every
time, by inverting this to a disable setting it works. To make trailer
trash play nice with unite you can put the following in your vimrc:

    function! s:unite_settings()
      let b:disable_trailertrash=1
    endfunction
Merge pull request #16 from csexton/disable-buffer

Add buffer var to disable displaying trailing spaces