Simpler computation/io naming for Schedulers

- simpler (no one remembers the current names when talking about them)
- does not tie naming to a particular implementation involving thread pools versus a pool of event loops or something similar (as we likely will change the implementation, see https://github.com/Netflix/RxJava/issues/713)
Merge pull request #725 from benjchristensen/schedulers-naming

Simpler computation/io naming for Schedulers