clarify interaction component id to avoid type-practice conflicts making it impossible to use and still be valid
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
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