Use microsoft server time in signed requests

[HashimTheArab]

• Saves microsoft server time from responses and uses it in signed requests, many user devices may have out of sync time which will cause a 403 forbidden error from xbox api
• Changes "deviceToken" key to "DeviceToken" to match real request
• Simplifies w.Write to fmt.Fprintf

SImply saving serverTime to a variable works because Token() is always called during the dial process before sending the signed requests obtainXblToken and obtainDeviceToken