simplified Logger with ServerEngine v1.5.6
redesigned bundler injection
redsigned process model using using ServerEngine::ProcessManager#spawn
Engine replaced worker_global_methods
Simple RootAgent::EventCollectLoggerMixin replaced StatsCollector
fixed Logger
removed Engine.root_agent
changed module EventEmitter to class EmitterAgent
merged EmitterAgent into Agent
log and error labels should not cause infinite loop
moved default_collector= invoke to PluginRegistry
Large refactoring.

* Added descriptive comments to major classes.
* Added Label class and moved initialization code of <source> elements
  from Agent to Label.
* Renamed Collector#short_circuit to #match to simplify EventRouter.
* Updated coding style.
  * ':key => value' to 'key: value'
  * 'rescue' with '$!' to 'rescue => e'
  * 'require_relative' to 'require'
added 'type' plugins and moved Configurable.register_type to PluginRegistry#register_type
Agent has its own log level separated from the global logger #200
updated internal log stream handling
moved emit_error handling from Output to EventRouter so that it works when Output#emit is called directly, not through #emits
Agent: supports log_level parameter #200
added Agent#handle_emits_error and Agent#handle_emit_error
moved Plugin.register_type to Configurable.register_typ so that Agent can use config_param
implemented strict config type handler