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.
I am expecting a string and then splitting it on comma to make a array. Here `@user_excluded_tables` is a string.