Clarified format parameter type

use of {} in the list was confusing given that some properties can be JSON.
Merge pull request #267 from bscSCORM/format

Clarified format parameter type
Activity definition is optional

resolves: https://github.com/adlnet/xAPI-Spec/issues/150
Merge pull request #268 from bscSCORM/activity_definition

Activity definition is optional
Merge pull request #12 from adlnet/master

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