Multi periods

[martinlindhe]

Hello and thanks for a cool project!

This PR adds support for multiple periods.
It extends upon the existing API, so it should not require any changes to existing client code.

Also included:

• xml marshaller for the xml duration type "PT6M" = 6 minutes
• an ExampleAddNewPeriod to show usage
• removed internal pointers to *MPD, simplified some.
• moved "invalid state" checks to separate func (m *MPD) Validate()
• minor cleanup

example usage for multiple periods

	// a new MPD is created with a single Period
	m := NewMPD(DASH_PROFILE_LIVE, VALID_MEDIA_PRESENTATION_DURATION, VALID_MIN_BUFFER_TIME)

	as, _ := m.AddNewAdaptationSetVideo(DASH_MIME_TYPE_VIDEO_MP4, VALID_SCAN_TYPE, VALID_SEGMENT_ALIGNMENT, VALID_START_WITH_SAP)
	as.SetNewSegmentTemplate(1968, "$RepresentationID$/video-1.mp4", "$RepresentationID$/video-1/seg-$Number$.m4f", 0, 1000)

	as, _ = m.AddNewAdaptationSetAudio(DASH_MIME_TYPE_AUDIO_MP4, VALID_SEGMENT_ALIGNMENT, VALID_START_WITH_SAP, VALID_LANG)
	as.SetNewSegmentTemplate(1968, "$RepresentationID$/audio-1.mp4", "$RepresentationID$/audio-1/seg-$Number$.m4f", 0, 1000)

	// add a second period
	p := m.AddNewPeriod()
	p.SetDuration(3 * time.Minute)
	// you can add content to the Period
	as, _ = p.AddNewAdaptationSetVideo(DASH_MIME_TYPE_VIDEO_MP4, VALID_SCAN_TYPE, VALID_SEGMENT_ALIGNMENT, VALID_START_WITH_SAP)
	as.SetNewSegmentTemplate(1968, "$RepresentationID$/video-2.mp4", "$RepresentationID$/video-2/seg-$Number$.m4f", 0, 1000)

	// or directly to the MPD, which will use the current Period.
	as, _ = m.AddNewAdaptationSetAudio(DASH_MIME_TYPE_AUDIO_MP4, VALID_SEGMENT_ALIGNMENT, VALID_START_WITH_SAP, VALID_LANG)
	as.SetNewSegmentTemplate(1968, "$RepresentationID$/audio-2.mp4", "$RepresentationID$/audio-2/seg-$Number$.m4f", 0, 1000)

[thomshutt]

Thanks @martinlindhe! We're going to try and review this soon