rewrote / restructured section 4.1.5 Result

line 981: changed from SHOULD to MUST; thought it was just a possible
reason for ambiguity and incompatibility if this is left to the
developer's choice.
Merge branch 'master' of https://github.com/adlnet/xAPI-Spec
corrected the details about 'scaled'

Based on this discussion https://github.com/adlnet/xAPI-Spec/pull/71
Merge pull request #71 from paul-esch/master

Rewrote section 4.1.5 Result