Added ability to manually get / set _transform.pan

I use this as a cheaper alternative to "proximity" which only seems worth using for looped sounds. I set the volume and pan of the sound based on distance from center of camera and then just play the sound, no overheads after the initial calculations.
Fixed getter.
Variable name change from _panning to _proximityPan to avoid confusion
Fixed getter.
Inlined get_pan / set_pan
Merge pull request #842 from PaulGene/patch-1

Added ability to manually get / set _transform.pan
Merge pull request #843 from SeiferTim/dev

Exposing FlxTween.duration
Update FlxTween subclasses for the _target -> duration change
Keyword order change:

static public var
-> public static var
Keyword order change:

inline public static var
-> public static inline var
Keyword order change:

inline static private var
-> private static inline var
Keyword order change:

static private var
-> private static var
Add keyword order change to the change log file.
Merge pull request #846 from HaxeFlixel/keyword_order

Keyword order
FlxAngle.getAngle(): remove Round param