Add .gemspec

Could you upload to rubygems.org?

    $ rake -T
    rake build    # Build fluent-plugin-groupcounter-0.1.0.gem into the pkg directory
    rake console  # Open an irb session preloaded with the gem library
    rake install  # Build and install fluent-plugin-groupcounter-0.1.0.gem into system gems
    rake release  # Create tag v0.1.0 and build and push fluent-plugin-groupcounter-0.1.0.gem to Rubygems