fix readme, font-weight consideration, body bug in ff & ie

The readme file was incorrect in description, because the default selector is '> div'. All examples with span children won't work with default settings.

Additionally I added font-weight to the considered css styles. Its value can change the width of text significantly.

Finally the check if(!'body' in document) results in FF and IE false, because the property is set but null. I changed it to !document.body.