Allow a nil starting value for -aggregateWithStart:reduce:

Fixes #1007.
Merge pull request #1074 from ReactiveCocoa/allow-nil-aggregate-start

Allow a nil starting value for -aggregateWithStart:reduce:
Merge remote-tracking branch 'origin/3.0-development' into fix-kvo-capturing-self
s/id/instancetype
Fix disposable usage errors in RACKVOWrapper