Fix Timeline::isComplete immediately after changes

The endTime is calculated based on the mDuration of this TimelineItem.  If the duration of this TimlineItem instance should be updated, then we should recalculate the duration before using mDuration in the calculation of endTime.
Merge pull request #515 from nhogle/patch-1

Fix Timeline::isComplete immediately after changes
defer throwing exception until IAudioClient::Initialize() fails. Fixes #518.
improve audio::Voice docs
Merge branch 'audio_wasapi_error_handling' into glNext