Update xAPI.md

Add vTrainingRoom to list of members of work group.
Added response code and Document API POST

Added a response code and message for the Document API POST.

Also made the state and profile api sections consistent with including POST.
Added case where original document doesn't exist

addressing #80
minor clarification when original document does not exist
Merge pull request #1 from bscSCORM/creighton

minor clarification when original document does not exist
Merge pull request #101 from creighton/master

Added a response code and message for the Document API POST.
Merge pull request #100 from vtrainingroom/master

Add member to the list
Activity and Verb Metadata

* changed metadata to JSON from XML within spec
* defined JSON format
* allowed any type of metadata at URI (LRMI, microdata, RDF, etc) (for activities, not listed, but implied)
* did the same for verb, etc... metadata -- except talking about "any format", do we want that here?
* ADD ‘url’ field to activity definition for ‘read more’ or ‘launch

Still todo:
TODO: 5 - create 2 content types (application/json+XXX)
TODO: 6 a- it may simply be a human readable document