Merge pull request #25 from adlnet/master

Sync up!
Update xAPI.md

5.1-5.3 - note State language removed from 5.1 to go generic across documents.  Requirement was "added" as it existed within the table.
5.3 saw non-requirement "shoulds" go into details, the rest separated into requirements.  Requirements around IRI "owners" and the "permission" were dropped in favor of language just saying that they should be controlled.  This "requirement" may be a bit much, but didn't want to risk moving to a non-patch version.
Update xAPI.md

4.4 up to requirements
Update xAPI.md

5.4 requirements done, except the last requirement which should be clarified.
Update xAPI.md

Section 5 - Typos corrected, some requirements removed/editted
Update xAPI.md

Attempt on the document key requirement
Update xAPI.md

Updated to remove requirement (from the section, still exists in other language elsewhere)
Merge pull request #355 from andyjohnson/master

Section 5 Edits/Formatting