Defaulting to utf-8 when chardet returns None

On articles like this one chardet returns None:
http://news.zing.vn/nhip-song-tre/thay-giao-gay-sot-tung-bo-luat-tinh-yeu/a291427.html
This causes exceptions later on when encoding.lower() is called
Merge pull request #56 from nathanathan/patch-1

Defaulting to utf-8 when chardet returns None