Bug fix for incorrect audio/video total index bug.

This bug #58 was due to the fact that getIsAudio function in DashManifestExtensions.js returns a promise while the calling function in DashMetricsExtensions.js was incorrectly treating the return value as boolean value.  Same is the case for getIsVideo.  Fixed it by adding a type field to the adaptation structure.  Please let me know if there is any other better way to achieve the same.