Check all FlxObjects for mouse events

Sprite groups treated as sprites + groups. Tilemaps are mouse-able now.
Best performance FlxPool

Array indexing is faster than any other proposed method.
Merge pull request #1254 from MSGhero/patch-1

Check all FlxObjects for mouse events, closes #1252
Merge pull request #1256 from MSGhero/patch-2

Best performance FlxPool, closes #1189
FlxObject: always init _point and _rect, #1250
FlxMouseEventManager: add setObjectMouseButtons()
Update CHANGELOG.md + minor cleanup
Compile fixes