Add the Kernel#singleton_class? predicate method

Hello,

This pull request implements the `#singleton_class?` predicate method. This is a convenient way to know whether a given object is a  class or not. This feature has been added in Ruby 2.1 (see https://bugs.ruby-lang.org/issues/7609 for further information).

A RubySpec has been added for that in rubyspec/rubyspec#279.

Have a nice day.