DoOn Tweaks

- do not have 2 method overloads with similar method signatures, dynamic languages can not negotiate method dispatch using function arity
- add doOnCompleted and doOnError methods instead of different doOnEach overloads
Merge pull request #483 from benjchristensen/onEach-fix

DoOn Tweaks
Remove a doOnEach overload

See https://github.com/Netflix/RxJava/pull/483 for more info