Merge pull request #8 from adlnet/master

updating
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
clarify attachments and behavior surrounding them
LRS Assuming Agents requirement
Merge branch 'master' into clarifyattachments
update attachment language for clarity
MUST NOT Trust application portion of authority
Merge pull request #280 from fugu13/clarifyattachments

Clarify Attachments
Updated Consumer key requirement
Merge pull request #271 from andyjohnson/master

4.1.7-4.1.9 Updates
Merge pull request #13 from adlnet/master

Sync up!
Updated Activity Definition and Added Client Def