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. 
This "requirement" has been moved to line 1549 and made a bit clearer.