allow empty log file, use the standard output in this case

Fixes #34

logger/sync_wrapper.go

@@ -0,0 +1,17 @@
+package logger
+
+import (
+	"os"
+	"sync"
+)
+
+type logSyncWrapper struct {
+	output *os.File
+	lock   *sync.Mutex
+}
+
+func (l logSyncWrapper) Write(b []byte) (n int, err error) {
+	l.lock.Lock()
+	defer l.lock.Unlock()
+	return l.output.Write(b)
+}