Fixed RunScript.hx, haxe3 + openFl support

This is a very useful tool which fixes an important NME/OpenFL flaw, and as it happens I just needed this right now for an assets-heavy project, so thanks a lot :)

However there were several issues to fix :
- using sys.FileSystem for Haxe 3
- using openl.Assets for OpenFl
- file types Regexes were wrong : for example ~/.+\.[(png)|(jpg)]/ would match asset.json which is not an image (character classes as well as capture groups were not needed here)
- paths in the xml were wrong, at least on my machine 
- AFAIK the option useFullPath was actually not implemented. I added a --relPath option which I found more clear since absolute paths are the default

Thanks !