Order MPD and Period elements per spec

[sbuzzard]

This orders the sub-elements of MPD and Period according to the DASH spec. Generally for most DASH consumers, this doesn't matter because even moderately modern XML parsers don't care about the ordering of independent elements in an XML hierarchy. However, there are systems using less evolved means to parse these messages and do care about the ordering of, in particular, MPD and Period, it turns out. And these systems, even when not using them, result in particular stringency in manual validation of an MPD by the spec. Technically, we'd probably want to do a scrub at some point for all elements; in the interim, this updates the order of MPD and Period elements based on latest spec. Understand that the changing of the order might require waiting to cut a 0.7.x, perhaps (though since the lib is still in a pre-1.0 state, perhaps not?). Appreciate you considering this. Thanks again for the lib - it's wonderful!

Fixes #75

[emarsden]

OK, thanks. I never imagined that order would be important, but it's naturally better to be spec compliant.

Currently away from home, won't be able to make a release until the weekend.