Fix FlxGroup sort() crash if it encounters another FlxGroup

If an object does not have the sorting property, it is placed at the end of the list.
Fix https://github.com/FlixelCommunity/flixel/issues/28 , https://github.com/AdamAtomic/flixel/issues/204
Code improvements and addition of CheckIndexExistence param.
Fix crash when there are null entries in the group.
Merge pull request #116 from Dovyski/fix_issue_28

Fix FlxGroup sort() crash if it encounters another FlxGroup or null entries.