Add timestamps to tusd log

[jjrodrig]

This PR adds timestamps to tusd logger.

The following line is modified https://github.com/tus/tusd/blob/master/config.go#L50 by setting the timestamp flags to the Log config

 if config.Logger == nil {
	config.Logger = log.New(os.Stdout, "[tusd] ", log.Ldate | log.Lmicroseconds)
 }

Flags log.Ldate | log.Lmicroseconds are used producing this kind of log line

 [tusd] 2019/05/24 11:39:18.827952 event="RequestIncoming" method="HEAD" path="..."

Closes #274

[Acconut]

Thank you very much 👍