Modifiers improved to be able to use them in more general pools :
 - reset methods taking a IEaseFunction as parameter added in BaseDoubleValueSpanModifier and BaseSingleValueSpanModifier classes
 - IModifier.clearModifierListeners method added to able to clear all modifier listeners when the modifier goes back in the pool
 - IModifier.onUnregister event method added to be able to recycle the modifier back to the pool easily
 - ModifierList.clear, remove and onUpdate method overriden from super class to call onUnregister event on modifiers
BufferedInputStream constructor called with 8192 as default size to avoid bug with old version of Android.
SurfaceGestureDetector.onFling method protected with NPE checks
Sprite.markTextureRegionChanged() method added to be able to modify a Sprite texture and to call onUpdateTextureCoordinates() in the Sprite.
Entity.toString method improved to avoid NPE
Entity.onManagedDraw and Entity.onManagedUpdate improved with try/catch to have more details on Exceptions
SpriteGroup.onUpdateSpriteBatch improved with try/catch in loop to have more details in case of Exception
BitmapTexture.java and classes extending IInputStreamOpener modified to have a more explicit NullBitmapException exception message.