Update to use RxJS instead of Highland
Expose shared instance of Rx on plumber for other modules to use

This is a workaround for https://github.com/Reactive-Extensions/RxJS/issues/187
Add serialFlatMap, fix parallelFlatMap for RxJS
Bump version to 0.4
Merge pull request #9 from plumberjs/move-to-rxjs

Move to rxjs