Separate validation from builders

[Wopple]

Currently some of the builders perform validation to ensure the data is representative of the specification. The responsibility for validation should be its own component. The builders should only be concerned with building the type.

[derjust]

Just as a bunch of items to consider (as just hit them - might be split up in dedicated issues when they are worked on):

• Check that TrackData has TrackInfo (Otherwise NullPointerException appears during writing)
  • IMHO already during building the object structure (covers read and in memory construction)
• Check TARGETDURATION against duration of the TrackData elements
  • IMHO during writing only. Maybe respect lenient vs. strict writing (compare to reading)
• Check that TrackData has a TrackInfo
  • IMHO already during building the object structure
• to be continued...

[Wopple]

WIP: https://github.com/iheartradio/open-m3u8/tree/validation