Add current value property to BehaviorSubject

[dalewking]

Rx 2.2 added a current value property that would come in handy. See discussion here: https://rx.codeplex.com/workitem/8

[akarnokd]

Hello! A PR is welcome, but note the following:

• Unlike Rx.NET, BehaviorSubject doesn't remember the very last onNext value before an onCompleted: completed subject emits only onCompleted to subscribers.
• The last known value is a NotificationLite object which might be null indicating a BehaviorSubject without default initial value.
• What should the method be called? value(), getValue(), current(), getCurrent() ?
• How should it propagate the various states: (empty, has value, has exception, terminated)? Perhaps via Notification?
• If it returns the last value directly, what exceptions should indicate empty and terminated states? NoSuchElementException?

[akarnokd]

Please check out #2470 for a set of Subject API changes that includes your current value.

[akarnokd]

Subjects have been enhanced in 1.0.5 with methods that get you their current state.