Merge pull request #8 from HaxeFlixel/dev

merge
FlxTypedEmitter: at() -> focusOn()

more descriptive and consistent with the FlxCamera API
Merge pull request #1070 from JoeCreates/dev

Allow camera bounds to be set individually and made null
FlxTypedEmitter: some cleanup, #1074

- made Bounds objects (default, null)
- moved getters / setters to the end
- inlined most getters / setters
- particleClass is no longer a property
FlxTypedEmitter: on -> emitting, closes #1076
FlxTypedEmitter tracker profile + Bounds.toString()
Update CHANGELOG.md
Move interfaces into their respective .hx files, closes #1080
PxButton.new(): OnClick should be Void->Void
FlxSpriteGroup only needs to be a typedef
Removed the plugin package, closes #1081

plugins are now in more meaningful places (tween manager in FlxTween.hx etc)
FlxTimerManager: no need for a seperate onStateSwitch()
Separate part of the util package into math, closes #1014
Next release is going to be 4.0.0
Move FlxTypedGroupIterator into FlxTypedGroup.hx
FlxTypedGroup.recycle(): some corrections for the docs
Fix FlxSubState.close() when called from create(
Merge pull request #1089 from Masadow/state

Fix FlxSubState.close() when called from create()
FlxSprite: better docs for framePixels
FlxPath: made nodeIndex public and read-only