Create a observable with the logic of userSignedIn() method #500

I'm submitting a...

• Regression (a behavior that used to work and stopped working in a new release)
• Bug report
• Performance issue
• Feature request
• Documentation issue or request
• Other... Please describe:

Current behavior

Actually userSignedIn() method can be used when is called only

Expected behavior

userSignedIn can be a observable, this ensures when the data changes to false, subscribers callbacks can be executed

What is the motivation / use case for changing the behavior?

In my app when user logout, is difficult handle with this method

Environment

Angular-Token version: ^7.0.1
Angular version: 7.1.4

Bundler

• Angular CLI (Webpack)
• Webpack
• SystemJS

Browser:

• Chrome (desktop) version XX
• Chrome (Android) version XX
• Chrome (iOS) version XX
• Firefox version XX
• Safari (desktop) version XX
• Safari (iOS) version XX
• IE version XX
• Edge version XX