Merge pull request #11 from adlnet/master

Sync up!
4.1.7 and 4.1.8

Timestamp definition updated for clarity (check please).  Re-formatted to be consistent with 4.1.x Sections.  Used "client" in place of reporting tool (check please).
Didn't realize it was Definition -> Description
Update xAPI.md
Merge pull request #12 from adlnet/master

Syncing
Update 4.1.9 Authority

Tried to break this out into requirements.  Any meaning change is unintentional, but want to gain clarity.  Most questionable is:

An LRS SHOULD assign each unregistered consumer a unique consumer key, which came from ", each unregistered consumer SHOULD pick a unique consumer key".  There was some debate on what this actually meant as an action to be done.
Russell/Tom changes
Group/Agent authority requirement update
Deleted redundant requirement
LRS Assuming Agents requirement
MUST NOT Trust application portion of authority
Updated Consumer key requirement
Merge pull request #271 from andyjohnson/master

4.1.7-4.1.9 Updates