Remove new line characters in parserDidEndDocument

Originally, `DTHTMLAttributedStringBuilder` used to leave the trailing new line character that was added during string construction. On iOS 8, if you assign the resulting attributed string to a UILabel, the trailing new line character is given a non-zero height (as expected), and thus the resulting text is not vertically centered. This change will most likely cause unit tests to fail. I haven't had a chance to delve into those.