Add speed value to GPX info

[Dwosky]

When querying a specific workout in /v1/sport/run/detail.json, aside the longitude_latitude, heart_rate, etc, there is also the speed parameter. Would it be possible to support that value as well on the exported gpx format?

If you don't have access to any workout with that value, basically its something like the following:

{
	"code": 1,
	"message": "success",
	"data": {
		[...]
		"speed": "1,4.76;0,4.65;1,4.65;0,4.60;[...];0,6.57;1,6.57;0,5.74",
		"cadence": ""
	}
}

[rolandsz]

It should be possible to use gpx extension to store the speed data, however, I think the most popular platforms (such as Strava) will recalculate it anyways based on the timestamps and points. If you had some problems with speed before, please check #13 which solves an issue related to timestamps.