It seems to be considered a single word. Not sure if these are valid references, but: 1. https://answers.yahoo.com/question/index?qid=20091003034818AAssnmO ht

It seems to be considered a single word. Not sure if these are valid references, but: 1. https://answers.yahoo.com/question/index?qid=20091003034818AAssnmO https://answers.yahoo.com/question/index?qid=20091003034818A... 2. http://en.wikipedia.org/wiki/Contraction_(grammar) http://en.wikipedia.org/wiki/Contraction_(grammar)