SDK / exoplayer

- Move parsing onto a background thread. This is analogous
  to how frame decoding is pushed to MediaCodec, and should
  prevent possible jank when new subtitle samples are parsed.
  This is more important for out-of-band subtitles, which can
  take a second or two to parse fully.

- Add Useful DataSpec method.

Issue: #12

Previously we'd end up blocking forever in this case, which
is the worst thing we could do :). We could either throw an
exception or just print a warning. Printing a warning is more
in line with what other methods do (Handler prints a "sending
message to dead thread" warning).

This allows ManifestFetcher to both execute the initial
manifest load and be plugged into an ExoPlayer ChunkSource,
where it can be used for repeated manfiest refreshes during
live playback.

Use locally bound variable instead of class variable.

Do this even if stop/disable fails.

Ignore the path of the base Uri if the reference starts with "/".
Spec - http://tools.ietf.org/html/rfc3986#section-5.2.2

This API wasn't particularly nice. Best to remove it whilst
hopefully no-one is using it. Leaving the ReadHead abstraction
in place, since it might well prove useful in the future.

Allow non-H264 adaptive decoders.

2. Common interface for manifest parsers.
- This effectively moves the common interface from the Fetcher level
  (i.e. ManifestFetcher) to the Parser level (i.e. ManifestParser).
- The motivation here is to allow the implementation of components that
  can work with a generic ManifestParser implementation.

- Skips unrecognized elements rather than crashing.
- FourCC treated as required for video and optional elsewhere,
  as per the SmoothStreaming spec.
- Only parse initData text when we're actually in the ProtectionHeader element

This means that after a decoder flush, the renderer will avoid
feeding non-keyframes into the decoder until it has received and
fed the first keyframe. The decoder has no way of correctly
decoding non-keyframes that arrive before a keyframe.

It looks like for the case of self-contained media segments,
it's possible to get stuck without failure in the case that
the load fails having loaded less than the length of the init
data.

Issue: #35