OBJExporter: handle children objects

+ scale factor param
loaders documentation: xhrloader, objloader, mtlloader
Cache, LoadingManager, OBJLoader, ObjectLoader, TextureLoader and XHRLoader documentation
add BabylonLoader documentation
Merge remote-tracking branch 'upstream/dev' into doc.loaders
OBJExporter: convert indents (spaces to tabs)
Merge remote-tracking branch 'upstream/dev' into doc.loaders
working on loaders docs, with new cross doc syntax
missing LoadingManager in some loaders
working on loaders docs, with new cross doc syntax
grammar errors
OBJExporter: remove useless check for object.children
OBJExporter: remove scaleFactor
OBJExporter: use object tag instead of group tag for object declaration
fix typo
add missing LoadingManager
new docs for some loaders
ColladaLoader doc
fix wrong Integer type
OBJExporter: move index vars in parse function
+ use object#traverse to parse each children