Fix FlxObject#(overlaps|overlapsAt) checking only first element of FlxGroup

FlxObject#overlaps and FlxObject#overlapsAt had a bug that would lead to only the first element of a FlxGroup being checked. It was happening because the variable "length" was not locally declared, however there was no compiling warns/errors because the Object class has a property named "length".
Tweaks for FlxObject.overlap* and fixes for FlxTilemap.overlap*

I extended the fixes of FlxObject.overlap*() to FlxTilemap.overlap*(). I also added a break statement when a collision is detected since all overlap() methods return just a boolean.

Fix https://github.com/FlixelCommunity/flixel/issues/39 and https://github.com/AdamAtomic/flixel/issues/192 .
Small code refactoring.
Merge pull request #2 from Dovyski/fix_issue_9_and_54

Fix issue 9 and 54