We no longer have to rely on browser support for a codec to check
Streamer support for a codec.  We can check the manifest right after
parsing, and ignore any playback errors.

This also adds a test case for PR shaka-project#74 (HEVC support) which was too
difficult to add before this refactor.

Change-Id: I37edb8ff7c94134e74e7a86f85c5b44bf4b38e24

Restrict WebM formats to DASH protocol only, preventing WebM formats in HLS.  Restriction is set via Shaka Packager's "dash_only" flag as per discussion in the Issue 18 thread.

Closes shaka-project#18

Changes:
  - Added `multiperiod_inputs_list` to parallel `inputs` for multi-period content
  - `ConflictingFields` error raised when `inputs` and `multiperiod_inputs_list` fields are present at the same time in the the input config (in `InputConfig.__init__()`)
  - `MissingRequiredExclusiveFields` error raised when neither `inputs` field nor `multiperiod_inputs_list` field is present (in `InputConfig.__init__()`)
  - `SinglePeriod` class added to represent each period in the input config
  - Refactored `controller.start()` to append more Transcoder and Packager nodes
  - Added `PeriodConcatNode` in `periodconcat_node.py`, a new `ThreadedNode` that is appended after all the Transcoder and Packager nodes.  If all of them are finished, it starts the period concatenation.  For DASH, the `xml.etree` module is used to parse the `.mpd` files and extract the periods and duration information.  For HLS, we might use `m3u8`.
  - In `ProcessStatus`, `Running` is assigned to a greater value than `Finished` to change the way we wait for all processes to complete, with exceptions made in `CloudNode` and `PeriodConcatNode`

Issue shaka-project#43

If required output framerate is < input source frame rate, adjust configuration accordingly to automatically reduce the frame rate.

faster vp9 encoding, whilst still maintaining balance between speed/quality