[TIMOB-10212] Add readonly duration property to iOS AudioPlayer

As per TIMOB-20212 (https://jira.appcelerator.org/browse/TIMOB-10212) add the duration property to the iOS Ti.Media.AudioPlayer proxy. Read-only and can only be read on > iOS 3.2. Manually merged based on changes by gerry3 in the #86 PR.

Will also be opening a separate PR for the Android equivalent. 

Getting track duration is a prerequisite to other features such as track seeking.