Update Observable.java

standardizing javadoc comments & adding wiki/image links for new methods
Update Observable.java

improve javadocs, including diagrams and wiki links for new operators
Missing ! .... grrr.
Fix bad unit test

- no further events expected after unsubscribe
Calling constructor explicitly on schedulers
Notification constructors
Implicit conversions
Add License Headers
Merge pull request #529 from benjchristensen/scala-tweaks

Scala Tweaks
Merge pull request #527 from Netflix/docs

Docs
[Gradle Release Plugin] - pre tag commit:  '0.15.1'.
[Gradle Release Plugin] - new version commit:  '0.15.2-SNAPSHOT'.
Version 0.15.1