Refactoring to leverage Comparator Framework

[sebastianbergmann]

PHPUnit 3.6 introduced a comparator framework that allows extensions to "overload" assertEquals() et. al. with custom comparison operations for objects. DbUnit should be refactored to leverage this by providing an implementation of PHPUnit_Framework_Comparator that can compare PHPUnit_Extensions_Database_DataSet objects and "just" provide convenience functionality to read data sets from CSV, XML, YAML, etc. as well as from a PDO connection.