startup: fixMayapy2011SegFault - seg fault was fixed in 2013
get rid of util.common.isClassRunningStack/Frame - calling inspect.stack() would cause maya to crash in some cases...
test_system: oops, restore the tearDown
removeReferences doesn't fail if the ref is loaded (as I thought it would)
system: clarified doc not about removeReferenceEdits not erroring
pmel.core.parent and DagNode.setParent now share common codebase