Update README to reflect repository move.
Replacing Netflix/RxJava with ReactiveX/RxJava
copying over recent javadoc enhancements from the 0.20 branch
#1607 wiping the javadoc @since slate clean for 1.0
fix the usage of deprecated methods.
Migrate Build to Nebula

Using plugin from https://github.com/nebula-plugins/gradle-rxjava-project-plugin