Run gofmt and add test to prevent regressions

Run `go fmt ./...` [1] and add a test to the `test` make target that
checks if `go fmt` has been run.

[1]: http://blog.golang.org/go-fmt-your-code

Makefile

@@ -9,6 +9,7 @@ deps:
 	go get -t -v ./...
 
 test:
+	@test -z "$(shell find . -name '*.go' | xargs gofmt -l)" || (echo "Need to run 'go fmt ./...'"; exit 1)
 	go vet ./...
 	go test -cover -short ./...