UTCTiming schemeIdUri and value pairs can be provided to packager using
--utc_timings flag. It should be comma separated list of
schemeIdUri=value pairs.
Note that urn:mpeg:dash:utc:direct:2014 scheme is not supported as it
requires the MPD to be dynamically generated on the fly when MPD is
served to client.
Fixes#311.
Change-Id: Ibc07af8a6d8b2b6261ba3ecd2c02f23809f96614