DOI has nothing to do with 'citable'

See http://carlboettiger.info/2013/06/03/DOI-citable.html for more. I've been guilty of propagating this myth myself in the past. We really need to avoid writing this. Anything can be cited, regardless of whether it has a DOI or not.
Merge pull request #37 from rossmounce/patch-33

DOI has nothing to do with 'citable'