Stack traces that span inter-actor calls

Previously Celluloid gave some hard-to-interpret stack traces. The new
stack traces span calls across actors. Here's an example:

	/Users/tony/src/celluloid/spec/support/actor_examples.rb:203:in `receiver_method'
	/Users/tony/src/celluloid/lib/celluloid/calls.rb:24:in `public_send'
	/Users/tony/src/celluloid/lib/celluloid/calls.rb:24:in `dispatch'
	/Users/tony/src/celluloid/lib/celluloid/actor.rb:326:in `block in handle_message'
	/Users/tony/src/celluloid/lib/celluloid/tasks/task_fiber.rb:24:in `block in initialize'
	(celluloid): remote procedure call
	/Users/tony/src/celluloid/lib/celluloid/actor.rb:70:in `call'
	/Users/tony/src/celluloid/lib/celluloid/proxies/actor_proxy.rb:106:in `method_missing'
	/Users/tony/src/celluloid/spec/support/actor_examples.rb:211:in `caller_method'
	/Users/tony/src/celluloid/lib/celluloid/calls.rb:24:in `public_send'
	/Users/tony/src/celluloid/lib/celluloid/calls.rb:24:in `dispatch'
	/Users/tony/src/celluloid/lib/celluloid/actor.rb:326:in `block in handle_message'
	/Users/tony/src/celluloid/lib/celluloid/tasks/task_fiber.rb:24:in `block in initialize'
	(celluloid): remote procedure call
	/Users/tony/src/celluloid/lib/celluloid/actor.rb:80:in `call'
	/Users/tony/src/celluloid/lib/celluloid/proxies/actor_proxy.rb:106:in `method_missing'
	/Users/tony/src/celluloid/spec/support/actor_examples.rb:217:in `block (3 levels) in <top (required)>'
	/Users/tony/.rvm/gems/ruby-1.9.3-p327/gems/rspec-core-2.12.2/lib/rspec/core/example.rb:114:in `instance_eval'
	/Users/tony/.rvm/gems/ruby-1.9.3-p327/gems/rspec-core-2.12.2/lib/rspec/core/example.rb:114:in `block in run'
	/Users/tony/.rvm/gems/ruby-1.9.3-p327/gems/rspec-core-2.12.2/lib/rspec/core/example.rb:254:in `with_around_each_hooks'
	/Users/tony/.rvm/gems/ruby-1.9.3-p327/gems/rspec-core-2.12.2/lib/rspec/core/example.rb:111:in `run'
	/Users/tony/.rvm/gems/ruby-1.9.3-p327/gems/rspec-core-2.12.2/lib/rspec/core/example_group.rb:388:in `block in run_examples'
	/Users/tony/.rvm/gems/ruby-1.9.3-p327/gems/rspec-core-2.12.2/lib/rspec/core/example_group.rb:384:in `map'
	/Users/tony/.rvm/gems/ruby-1.9.3-p327/gems/rspec-core-2.12.2/lib/rspec/core/example_group.rb:384:in `run_examples'
	/Users/tony/.rvm/gems/ruby-1.9.3-p327/gems/rspec-core-2.12.2/lib/rspec/core/example_group.rb:369:in `run'
	/Users/tony/.rvm/gems/ruby-1.9.3-p327/gems/rspec-core-2.12.2/lib/rspec/core/example_group.rb:370:in `block in run'
	/Users/tony/.rvm/gems/ruby-1.9.3-p327/gems/rspec-core-2.12.2/lib/rspec/core/example_group.rb:370:in `map'
	/Users/tony/.rvm/gems/ruby-1.9.3-p327/gems/rspec-core-2.12.2/lib/rspec/core/example_group.rb:370:in `run'
	/Users/tony/.rvm/gems/ruby-1.9.3-p327/gems/rspec-core-2.12.2/lib/rspec/core/example_group.rb:370:in `block in run'
	/Users/tony/.rvm/gems/ruby-1.9.3-p327/gems/rspec-core-2.12.2/lib/rspec/core/example_group.rb:370:in `map'
	/Users/tony/.rvm/gems/ruby-1.9.3-p327/gems/rspec-core-2.12.2/lib/rspec/core/example_group.rb:370:in `run'
	/Users/tony/.rvm/gems/ruby-1.9.3-p327/gems/rspec-core-2.12.2/lib/rspec/core/command_line.rb:28:in `block (2 levels) in run'
	/Users/tony/.rvm/gems/ruby-1.9.3-p327/gems/rspec-core-2.12.2/lib/rspec/core/command_line.rb:28:in `map'
	/Users/tony/.rvm/gems/ruby-1.9.3-p327/gems/rspec-core-2.12.2/lib/rspec/core/command_line.rb:28:in `block in run'
	/Users/tony/.rvm/gems/ruby-1.9.3-p327/gems/rspec-core-2.12.2/lib/rspec/core/reporter.rb:34:in `report'
	/Users/tony/.rvm/gems/ruby-1.9.3-p327/gems/rspec-core-2.12.2/lib/rspec/core/command_line.rb:25:in `run'
	/Users/tony/.rvm/gems/ruby-1.9.3-p327/gems/rspec-core-2.12.2/lib/rspec/core/runner.rb:80:in `run'
	/Users/tony/.rvm/gems/ruby-1.9.3-p327/gems/rspec-core-2.12.2/lib/rspec/core/runner.rb:17:in `block in autorun