Prevent double creation of the volume slider

When the iOS App reload after background, prevent to create another instance of the VolumeSlider
update to cordova 3.x
Fix: add MediaPlayer Framework ref
update objective C for new cordova signature and fix ARC error
Fix bug and use better syntax
Fix a small bug (take into account height param)
Updated to 3.0 compatibly by @orbitaloop
Merge pull request #4 from devgeeks/3.0-compatibility

3.0 compatibility