parse and expose level name in Level.as

fix https://github.com/mangui/HLSprovider/issues/93
it is working fine with JW6
However GrindPlayer does not use level name but bitrate/height (refer to https://github.com/kutu/GrindFramework/blob/master/src/ru/kutu/grind/views/mediators/QualityMenuBaseMediator.as#L145)