Provide option to force use of default font for symbols

On my Mac, this seems to be necessary, otherwise Emacs always picks a
different font for symbols. That font inevitably has a different width,
so the resulting rendering is ugly.
Make ac-slime work with TAB in the slime REPL
Merge branch 'purcell'

* purcell:
  Make ac-slime work with TAB in the slime REPL
  Provide option to force use of default font for symbols
Drop pointback (see #133)
Don't show trailing whitespace in twittering-mode, if installed
Merge branch 'purcell'

* purcell:
  Don't show trailing whitespace in twittering-mode, if installed
  Drop pointback (see #133)