Confusing Javadoc for `toObservable(Future)` methods

https://github.com/Netflix/RxJava/issues/148
Merge pull request #187 from benjchristensen/issue-148

Confusing Javadoc for `toObservable(Future)` methods
Version 0.6.0

This release includes some small breaking changes to the API.
Subscriptions cleanup

- make createSubscription be create … as I had originally intended to do but forgot to
- make EmptySubscription a singleton
update clojure and groovy code to subscriptions changes
Merge pull request #188 from benjchristensen/issue-173

Subscriptions cleanup