Initial addition of onStart

Also renamed "complete" property to "onComplete"
Change indentation to tabs to conform to style guide
Merge branch 'dev' into feature-tween-onstart

* dev:
  Add FlxKey.toString(), closes #1258
Corrected outdated documentation
Updated change log
Added an onUpdate callback function to FlxTween
FlxG.switchState: added conditional check for flixel_addons that handles FlxTransitionStates cleanly
added FillStyle back to make flixel-addons compile again (needed for shapes)
Cleaned up FlxG.switchState transition a bit.
Merge pull request #1273 from anissen/feature-tween-onstart

Added onStart parameter to FlxTween
Update FlxTween.hx
Transitions: removed FlxG.switchState hack for something a bit more elegant, added lightweight transition API to FlxState
FlxTween: fix not being able to pause a looping tween in onComplete()
Update CHANGELOG.md
FlxTimer#complete -> onComplete

consistency with FlxTween, FlxPath and FlxSound
Merge pull request #1275 from Gama11/FlxTimerOnComplete

FlxTimer#complete -> onComplete
FlxTween: initialize scale with 0 to prevent NaN errors (#1273)
Transitions: removed FillStyle from my branch now that it's no longer necessary
Restore immovable value after Motion tweens and FlxPaths, closes #1279

+ less redundancy in FlxTween#finish()
FlxAssets#getFileReferences(): ingore invisible files, closes #1280