Add failing spec for supervision group tree

Tested against JRuby 1.7.8 and 1.7.9, in both cases the actor supervised by a child supervision group doesn't seem be started, and isn't in `Celluloid::Actor.all`. Tested against b9c73fd1


Celluloid::SupervisionGroup
  runs applications
  accepts a private actor registry
  removes actors from the registry when terminating
  pool
    runs applications and passes pool options and actor args
    allows external access to the internal registry
  supervision group tree
    starts child-supervised actors (FAILED - 1)

Failures:

  1) Celluloid::SupervisionGroup supervision group tree starts child-supervised actors
     Failure/Error: Celluloid::Actor[:example].should be_running
     NoMethodError:
       undefined method `running?' for nil:NilClass
     # /home/doug/.gem/jruby/1.9.3/gems/rspec-expectations-2.14.4/lib/rspec/matchers/built_in/be.rb:145:in `matches?'
     # /home/doug/.gem/jruby/1.9.3/gems/rspec-expectations-2.14.4/lib/rspec/expectations/handler.rb:24:in `handle_matcher'
     # /home/doug/.gem/jruby/1.9.3/gems/rspec-expectations-2.14.4/lib/rspec/expectations/syntax.rb:53:in `should'
     # ./spec/celluloid/supervision_group_spec.rb:81:in `(root)'
     # org/jruby/RubyBasicObject.java:1536:in `instance_eval'
     # /home/doug/.gem/jruby/1.9.3/gems/rspec-core-2.14.7/lib/rspec/core/example.rb:114:in `run'
     # org/jruby/RubyProc.java:271:in `call'
     # /home/doug/.gem/jruby/1.9.3/gems/rspec-core-2.14.7/lib/rspec/core/example.rb:179:in `run'
     # ./spec/spec_helper.rb:35:in `(root)'
     # org/jruby/RubyBasicObject.java:1565:in `instance_exec'
     # /home/doug/.gem/jruby/1.9.3/gems/rspec-core-2.14.7/lib/rspec/core/extensions/instance_eval_with_args.rb:16:in `instance_eval_with_args'
     # /home/doug/.gem/jruby/1.9.3/gems/rspec-core-2.14.7/lib/rspec/core/example.rb:247:in `instance_eval_with_args'
     # /home/doug/.gem/jruby/1.9.3/gems/rspec-core-2.14.7/lib/rspec/core/hooks.rb:106:in `run'
     # org/jruby/RubyProc.java:271:in `call'
     # /home/doug/.gem/jruby/1.9.3/gems/rspec-core-2.14.7/lib/rspec/core/example.rb:179:in `run'
     # ./spec/spec_helper.rb:30:in `(root)'
     # org/jruby/RubyBasicObject.java:1565:in `instance_exec'
     # /home/doug/.gem/jruby/1.9.3/gems/rspec-core-2.14.7/lib/rspec/core/extensions/instance_eval_with_args.rb:16:in `instance_eval_with_args'
     # /home/doug/.gem/jruby/1.9.3/gems/rspec-core-2.14.7/lib/rspec/core/example.rb:247:in `instance_eval_with_args'
     # /home/doug/.gem/jruby/1.9.3/gems/rspec-core-2.14.7/lib/rspec/core/hooks.rb:106:in `run'
     # org/jruby/RubyProc.java:271:in `call'
     # /home/doug/.gem/jruby/1.9.3/gems/rspec-core-2.14.7/lib/rspec/core/hooks.rb:104:in `run'
     # /home/doug/.gem/jruby/1.9.3/gems/rspec-core-2.14.7/lib/rspec/core/hooks.rb:446:in `run_hook'
     # /home/doug/.gem/jruby/1.9.3/gems/rspec-core-2.14.7/lib/rspec/core/example_group.rb:340:in `run_around_each_hooks'
     # /home/doug/.gem/jruby/1.9.3/gems/rspec-core-2.14.7/lib/rspec/core/example.rb:256:in `with_around_each_hooks'
     # /home/doug/.gem/jruby/1.9.3/gems/rspec-core-2.14.7/lib/rspec/core/example.rb:111:in `run'
     # /home/doug/.gem/jruby/1.9.3/gems/rspec-core-2.14.7/lib/rspec/core/example_group.rb:390:in `run_examples'
     # org/jruby/RubyArray.java:2409:in `map'
     # /home/doug/.gem/jruby/1.9.3/gems/rspec-core-2.14.7/lib/rspec/core/example_group.rb:386:in `run_examples'
     # /home/doug/.gem/jruby/1.9.3/gems/rspec-core-2.14.7/lib/rspec/core/example_group.rb:371:in `run'
     # /home/doug/.gem/jruby/1.9.3/gems/rspec-core-2.14.7/lib/rspec/core/example_group.rb:372:in `run'
     # org/jruby/RubyArray.java:2409:in `map'
     # /home/doug/.gem/jruby/1.9.3/gems/rspec-core-2.14.7/lib/rspec/core/example_group.rb:372:in `run'
     # /home/doug/.gem/jruby/1.9.3/gems/rspec-core-2.14.7/lib/rspec/core/command_line.rb:28:in `run'
     # org/jruby/RubyArray.java:2409:in `map'
     # /home/doug/.gem/jruby/1.9.3/gems/rspec-core-2.14.7/lib/rspec/core/command_line.rb:28:in `run'
     # /home/doug/.gem/jruby/1.9.3/gems/rspec-core-2.14.7/lib/rspec/core/reporter.rb:58:in `report'
     # /home/doug/.gem/jruby/1.9.3/gems/rspec-core-2.14.7/lib/rspec/core/command_line.rb:25:in `run'
     # /home/doug/.gem/jruby/1.9.3/gems/rspec-core-2.14.7/lib/rspec/core/runner.rb:80:in `run'
     # /home/doug/.gem/jruby/1.9.3/gems/rspec-core-2.14.7/lib/rspec/core/runner.rb:17:in `autorun'

Finished in 0.242 seconds
6 examples, 1 failure

Failed examples:

rspec ./spec/celluloid/supervision_group_spec.rb:77 # Celluloid::SupervisionGroup supervision group tree starts child-supervised actors