introduce unused

Author: haya14busa

.travis.yml

@@ -8,9 +8,11 @@ install:
   - go get -d -v -t ./...
   - go get github.com/mattn/goveralls
   - go get github.com/golang/lint/golint
+  - go get honnef.co/go/unused/cmd/unused
 
 script:
   - go test -v -race ./...
   - goveralls -service=travis-ci
   - go vet ./...
   - golint -set_exit_status .
+  - unused ./...