Refactor rx.concurrency to rx.schedulers

To match the naming convention of rx.observables, rx.subjects and rx.subscriptions we have decided to rename rx.concurrency to rx.schedulers prior to the 1.0 release. These commits makes this change.

Since this is a non-trivial breaking change to anyone using Schedulers, this leaves the `rx.concurrency.*` classes in place and deprecates them to give time to migrate. These deprecated classes will be deleted prior to the 1.0 release.