Refactored rake task into a class

 - Added dumper class and moved all the logic into that class.
 - fixtures_path method is not present in Rails 3 and requires Active
   Record's 'config_load' task to be run. So removed using it in
   fixtures_path method.
How about a simpler name something like `IGNORE_TABLES` or `EXLUDE_TABLES`.

In the current name I'm not sure what system means. For a while I thought these are the tables which are maintained internally by postgresql or mysql.