Merge HLS playlist parsers, make a single parser identify the
playlist type (master or media).

Issue: #155
Improve HLS ABR.

- Add options to switch abruptly at segment boundaries. Third
  parties who guarantee keyframes at the start of segments will
  want this, because it makes switching more efficient and hence
  rebuffering less likely.
- Switch quality faster when performing a splicing switch (when
  we detect that we need to switch variant, we now immediately
  request the same segment as we did last time for the new variant,
  rather than requesting one more segment for the old variant
  before doing this.