Created the initial implementation of BehaviorSubject
Improved documentation of the BehaviourSubject.
Merge branch 'subject-implementations' of git://github.com/michaeldejong/RxJava into pull-256-merge-BehaviorSubject
Removing SynchronizedObserver usage from Subject implementations.

- We don't need to add synchronization as the subjects can trust their source Observables to comply with the Rx contract.
- This optimization follows Rx Design Guidelines 6.8. Avoid serializing operators

This was discussed at https://github.com/Netflix/RxJava/pull/256
Merge pull request #264 from benjchristensen/pull-256-merge-BehaviorSubject

Merge BehaviorSubject from Pull 256
Gradle Release Plugin - pre tag commit:  'rxjava-0.8.2'.