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 was my attempt to preserve the descriptive 'should' for 1.0.3 however we have made 'should' into MUST elsewhere on the basis that descriptive language is still enforceable and that i'm not aware of any adoption to the contrary. I felt we had to require the LRS to set it to something as a MUST as to not do so would probably be effectively a breaking change for some reporting tool. 

Happy to merge these two lines to  ``` * MUST be set by the LRS to the value of [Stored](#stored) if not provided. ```

Can somebody else either +1 or -1 this suggested change to this PR before I do so?