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. 
Based on the original text, it seems permissible to leave the timestamp alone, or to set it to the same value as 'stored'. Though really, this should have been a must in the first place. so:

<pre>SHOULD* be set by the LRS to the value of [Stored](#stored) if not provided.</pre>