Alpha Hotfix Release 0.5.1

This is a hotfix for 0.5.0 that addresses the following:

• Fixed an issue with Audio.length always returning 0, instead of the audio length in seconds.
• Amended the Audio.pan, Audio.pitch and Audio.position properties to get/set as doubles instead of floats, to match Lua's usage of double.
• Added an Audio.cursor property to get the current position of an Audio resource, in seconds.
• Added an Audio.volume property to get/set the volume of an Audio resource.