use a class instance variable instead of a class variable

I replaced the @@default_logger class variable with a @default_logger class instance variable.  This resolves the Ruby 2.0.0 "class variable access from toplevel" warning ( https://github.com/ib-ruby/ib-ruby/issues/75 ).  I also tested the logger under Ruby 1.9.3; it still works fine with this change.  See also http://thinkrelevance.com/blog/articles/2006/11/16/use-class-instance-variables-not-class-variables