Removes the ambiguous word "blank" in 3 places.

Fix https://github.com/adlnet/xAPI-Spec/issues/471#issuecomment-67382428 as agreed on yesterdays call. 

Also addresses two other occurrences of the word "blank" in the spec. 

Also tidies up some lowercase shoulds around timestamp and authority. 
So the LRS could set it to some other value if not provided? Seems very strange. If we're going to require it be set if not provided, we should require it be set to stored in that case