Add lib/j_unit.rb to be able to omit the require 'yarjuf' statements

Without this I had to do `rspec -r yarjuf -f JUnit` or `bundle exec rspec -f JUnit` to load yarjuf **before** rspec was loaded. Otherwiese (e.g. putting `require 'yarjuf'`in a spec_helper.rb) errors like this were raised:

    dpree@pinguin ~/dev/test 1.9.3-p327 master $ rspec -f JUnit
    /Users/dpree/.rbfu/versions/1.9.3-p327/lib/ruby/gems/gems/rspec-core-2.13.1/lib/rspec/core/configuration.rb:1021:in `require': cannot load such file -- j_unit (LoadError)
	from /Users/dpree/.rbfu/versions/1.9.3-p327/lib/ruby/gems/gems/rspec-core-2.13.1/lib/rspec/core/configuration.rb:1021:in `rescue in custom_formatter'
	from /Users/dpree/.rbfu/versions/1.9.3-p327/lib/ruby/gems/gems/rspec-core-2.13.1/lib/rspec/core/configuration.rb:1018:in `custom_formatter'
	from /Users/dpree/.rbfu/versions/1.9.3-p327/lib/ruby/gems/gems/rspec-core-2.13.1/lib/rspec/core/configuration.rb:507:in `add_formatter'
	from /Users/dpree/.rbfu/versions/1.9.3-p327/lib/ruby/gems/gems/rspec-core-2.13.1/lib/rspec/core/configuration_options.rb:30:in `block in configure'
	from /Users/dpree/.rbfu/versions/1.9.3-p327/lib/ruby/gems/gems/rspec-core-2.13.1/lib/rspec/core/configuration_options.rb:30:in `each'
	from /Users/dpree/.rbfu/versions/1.9.3-p327/lib/ruby/gems/gems/rspec-core-2.13.1/lib/rspec/core/configuration_options.rb:30:in `configure'
	from /Users/dpree/.rbfu/versions/1.9.3-p327/lib/ruby/gems/gems/rspec-core-2.13.1/lib/rspec/core/command_line.rb:21:in `run'
	from /Users/dpree/.rbfu/versions/1.9.3-p327/lib/ruby/gems/gems/rspec-core-2.13.1/lib/rspec/core/runner.rb:80:in `run'
	from /Users/dpree/.rbfu/versions/1.9.3-p327/lib/ruby/gems/gems/rspec-core-2.13.1/lib/rspec/core/runner.rb:17:in `block in autorun'


After the change I made you can just do `rspec -f JUnit` without doing any pre-require statements.