Support self-contained media chunks.

- Support parsing of moov atoms contained within each chunk.
- Also do a small cleanup to WebM parser.
Use bits/sec instead of bytes/sec for format bandwidth.

Why: This was a bad initial choice. Manifests typically define bandwidth in
bits/sec. If you divide by 8 then you're throwing away information due to
rounding. Unfortunately it turns out that SegmentTemplate based manifests
require you to be able to recall the bitrate exactly (because it's substituted
in during segment URL construction).

Medium term: We should consider converting all our bandwidth estimation
over to bits/sec as well.

Note1: Also changed Period id to be a string, to match the mpd spec.
Note2: Made small optimization in FormatEvaluator to not consider discarding
the first chunk (durationBeforeThisSegmentUs will always be negative, and even
in the error case where it's not, removing the first thunk should be an error).
Add utility classes for SegmentTemplate/SegmentList refactor.

A step towards supporting SegmentTemplate style MPDs.
Define DashSegmentIndex wrapper.

This paves the way for SegmentTemplate and SegmentList based
mpds, which will implement DashSegmentIndex directly rather than
parsing an index from the media stream.

- Define DashSegmentIndex.
- Make use of DashSegmentIndex in chunk sources.
- Define an implementation of DashSegmentIndex that wraps a SegmentIndex.
- Add method that will allow Representations to return a DashSegmentIndex
  directly in the future.
- Add support for non-contiguous index and initialization data in media streams.
  For the Webm case this isn't enabled yet due to extractor limitations.
- Removed ability to fetch multiple chunks. This functionality does not extend
  properly to SegmentList and SegmentTemplate variants of DASH.
Add support for SegmentTemplate and SegmentList mpds.

Misc Notes:
- Removed content type filters because some of third parties
  don't set content type.
Bump version to 1.0.11.
Add some additional information to README.md.