Make sure SmoothStreaming manifest durations are -1 for Live.

Plus start to properly document the SmoothStreaming package.
Note that where the documentation is a little vague, this is
because the original SmoothStreaming documentation is equally
vague!
Make single MICROS_PER_SECOND constant + use it everywhere.
Fixed issue in which setting a representation duration to unknown wasn't handled correctly.
Audio improvements.
Implement DASH Live.

Note: This adds support for the majority of DASH live streams,
however we do not yet correctly support live streams that rely
on UtcTimingElements in their manifests.

Issue: #52
Merge branch 'dev' into dev-hls