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
Merge branch 'master' of https://github.com/adlnet/xAPI-Spec
clarify interaction component id to avoid type-practice conflicts making it impossible to use and still be valid
clarify attachments and behavior surrounding them
detail behavior of statement in context. This is the only change to the standard that breaks forwards compatibility, but as far as is known there are no non-test statements that use the broken behavior
clarify conversion behavior around context:statement
LRS Assuming Agents requirement
Merge pull request #279 from fugu13/interactioncomponentidfix

Interaction Component ID Fix
Merge pull request #281 from fugu13/contextstatement

Restrict context:statement to StatementRef
Merge branch 'master' of https://github.com/adlnet/xAPI-Spec
Merge branch 'master' into clarifyattachments
update attachment language for clarity