Add people.union

Support for $union operator, parallel to $append
Merge branch 'union' into develop
Override /track endpoint only if event is old
Merge branch 'track-import' into develop
add test for union
Merge branch 'union' into develop
test track vs import endpoints
Merge branch 'track-import' into develop
add people.update function & test
Merge branch 'update' into develop
Merge pull request #1 from uxnovo/develop

people.union & track/import fixes