Deadweight#strip strips whitespace

Selectors like `.foo ::-webkit-input-placeholder` were causing
Nokogiri to throw an exception. Specifically,
`Deadweight#strip` would remove the `::-webkit-input-placeholder`,
leaving a trailing whitespace, which Nokogiri doesn't like.
Merge pull request #28 from jamesarosen/james-strip

Deadweight#strip strips whitespace