DASH spec and the ordering of EventStream and other subelements

[sbuzzard]

Our architect who specializes in DAI pointed out that the order of the EventStream in a Period is not correct. Though most XML parsers don't care, they want us to abide by the spec. I can also do a PR to update this. Section 5.3.2.3 gives the order of the elements within the Period:

baseURL
segmentBase
segmentList
SegmentTemplate
AssetIdentifier
EventStream
AdaptationSet
Subset
SupplementalProperty
EmptyAdaptationSet
GroupLabel
Preselection

In that order. Some may not be supported and we don't care about them. I can create a PR with Period adjusted if you'd like.

Thanks so much for the lib!