Version 1.3 - RxJava Observable Integration

This pull request completes issue https://github.com/Netflix/Hystrix/issues/123 "Support Asynchronous Callbacks with RxJava Integration"

Async execution can now be done with `observe()` and it will callback when the value is received:

```java
Observable<String> s = new CommandHelloWorld("World").observe();
```

Here is the simplest possible example of subscribing to the value (using a lambda instead of anonymous inner class in this example):

```groovy
s.subscribe({ value -> println(value) })
```

More can be learned about RxJava and the composition features at https://github.com/Netflix/RxJava/wiki

This pull request is being canary tested at Netflix and will not be merged or released until we are comfortable with deploying this to our full production traffic. Since this is a non-trivial change I will probably leave it in our canary environment for around a week before proceeding (I have already done several smaller canary tests at various development stages). If any issues are found this pull request will be updated with the fix.

I am posting this pull request before our testing is complete to give everyone a heads up and an opportunity to provide feedback.