Elimination of Notice on empty utmb

Added isset conditional for utmb cookie to prevent notices & single-quoted string literals.

Rationale: If the class is called, and __utma and __utmz but NOT __utmb are set, the class throws a notice of an undefined index for $_COOKIE['__utmb]. This fix puts in place an isset check for $_COOKIE['__utmb'] before parsing the cookie.

I also added a micro-performance enhancement by switching double quotes to single quotes on string literals.

Thanks for the great work!