Add Managing Lifecycle State

[jraska]

Currently the library uses LiveData.observeForever(Observer) method and does not allow the clients to change the state of LiveData. This would be possible by using observe(Lifecycle, Observer)and adding lifecycle management to Observer API.

At the moment I'm not sure if adding this kind of management is worth the cost of increasing library scope, but I'm opening this based on Medium comment to try it and possibly gather more feedback.

Please comment with possible use cases or just thumbs up.

[jraska]

Option here: 24f7f4d

Problem is that doing this adds more methods, which are useless in case TestObserver instance is created on its own. We may restrict this, but it may piss off some users, who use the observer separately.