Spritekitreader: merged changes for CCBReader version 7

merged jtwigg's changes

I had to manually merge changes in these methods due to a conflict:
readNodeGraphParent:
nodeFromClassName:(NSString*)nodeClassName

Affects the way nodes are created from class name, it's no longer a 2-step process because the reader needs to be able to verify the class name and immediately create and return a new node based on the class name.