[Feature] Observable

[pepicrft]

What?

New observable feature. Get an Observable<T> entity from your storage given a Request<T>. That observable can be observed and every time the data affected by your request changes, you'll get notified using a closure.

Fixes #234

TODO

• Implement the feature including unit tests
• Update the CHANGELOG file
• Update the README file adding a section about Observables

It includes extensions for ReactiveCocoa & RxSwift