added 5.1.1 JSON Documents
Added POST activities/state

Needs tidying up clearly, but is this along the right lines?
create 7.3 Document APIs
replaced SHOULD with MUST for actions of the LRS
Clarified merge process
Clarified behavior if Content-Type: is not application/json, or if content is not valid JSON
Merge pull request #9 from bscSCORM/jsonDocuments

Some clarifications
changes as discussed https://github.com/adlnet/xAPI-Spec/pull/31
Merge pull request #31 from garemoko/jsonDocuments

JSON Documents