SchedulePeriodically Signature

This pull requests adds `TimeUnit delayUnit` to the signature:

```java
public Subscription schedulePeriodically(final Action1<Scheduler.Inner> action, long initialDelay, TimeUnit delayUnit, long period, TimeUnit periodUnit)
```

This was derived from feedback from @headinthebox while writing the Scala APIs.

Reason for ... 

- it better matches languages that have a single object that represent both together such as Scala.
- Java 8 adds `Duration`: http://download.java.net/jdk8/docs/api/java/time/Duration.html 

Reason against ... 

- Java 7 and earlier doesn't do it this way on their Executor.
- Java 8 despite adding `Duration` still doesn't use it on their `ScheduledExecutorService`

Thoughts, concerns, opinions?