PDFs in the wild can have a cmap bfrange containing > 255 characters

* I'm not sure why I added this check in the first place. Maybe because
  the cmap spec says only 255 chars are allowed. If that's the case I
  guess we follow the usual policy of "do what adobe acrobat does" and
  support ranges that aren't spec compliant.
* thanks to AJarreck for reporting it
* closes #87