Implement Label feature
Replace Engine.emit with router.emit
Add filter feature
match and filter should be initialized with mix order
Output provides router attribute to emit event stream
Fix to cannnot refer rear Label
Add relabel output plugin

From sonots's PR: https://github.com/fluent/fluentd/pull/417
Change Filter API

filter method should return filtered record and
default filter_stream behaviour now creates MultiEventStream at each call.
It accepts both MessagePackEventStream and other modifiable with same code.
Improvement @ERROR and logging handling
Fix test with EventRouter
Merge pull request #416 from fluent/add-filter-and-label

Add filter and label