Add register_xxx API to parser and formatter plugins
Merge pull request #495 from fluent/add-register-api-to-parser-and-formatter

Add register_xxx API to parser and formatter plugins
fixed bool_value and add test for Fluent::Config.size_value, time_value, bool_value
Merge pull request #477 from tatsu-yam/fix-bool_value

fixed bool_value
Add error stream API to EventRouter

With @ERROR, emitted events are routed to @ERROR label with warning message.
Without @ERROR, emitted events are dumpped in Fluentd's warning logs.
Use emit_error_event for each filter call
Add emit_error_event to RootAgentProxyWithoutErrorCollector
Merge pull request #492 from fluent/add-emit-error-event

Add error stream API to EventRouter