Release/2.4.0

@Raizlabs/maintainers-ios 

### Release Candidate 2.4.0

#### Features
- Add object being observed as parameter of KVO blocks in NSObject+RZBlockKVO
   - Deprecate the old signature as not to force a major version update

#### Fixes

- Fix race condition in RZDelayedOperation where cancellation can occur before operation block called with `dispatch_async` has a chance to execute

#### TODO
- [ ] Tag for 2.4.0
- [ ] Update and push podspec