#29.  Prevent fatal PHP error by checking for valid PDOStatement object.
 #29. Add an informative comment explaining PDO's unusual behavior with prepared statements.
 #29.  Check for error return from execute() before trying to call any version of fetch().  If there is an error, the return value is 'false' instead of a valid instance of PDOStatement.  Without these changes, errors cause fatal PHP errors when, for example, $this->fetch() is execute without $this being a valid object.
Merge pull request #34 from cxj/catcherrors

Suggested fixes to issue #29.