最近在研究 iOS 音樂剪輯／播放這一塊，紀錄一下
Trim :
http://stackoverflow.com/questions/13097331/unable-to-trim-a-video-using-avassetexportsession
http://stackoverflow.com/questions/9698787/ios-audio-trimming
http://stackoverflow.com/questions/21463250/cocoa-avasset-loaded-from-file-has-0-tracks
Export Mediaitem to local file:
http://stackoverflow.com/questions/13925148/exporting-video-from-users-itunes-library
http://stackoverflow.com/questions/4238219/trim-audio-with-ios
