ForEach aliases to Subscribe to match Java 8 Convention

As per discussion at #678

Code like this is now supported:

```java
Observable.from(1, 2, 3).forEach(System.out::println);
Observable.from(1, 2, 3).toBlocking().forEach(System.out::println);
```

Author: benjchristensen

rxjava-core/src/main/java/rx/Observable.java

@@ -3902,6 +3902,69 @@ public final <R> Observable<R> flatMap(Func1<? super T, ? extends Observable<? e
         return mergeMap(func);
     }
 
+    /**
+     * Subscribes to the {@link Observable} and receives notifications for each element.
+     * <p>
+     * Alias to {@link #subscribe(Action1)}
+     * 
+     * @param onNext
+     *            {@link Action1} to execute for each item.
+     * @throws IllegalArgumentException
+     *             if {@code onNext} is null
+     * @throws IllegalArgumentException
+     *             if {@code onError} is null
+     * @throws IllegalArgumentException
+     *             if {@code onComplete} is null
+     * @see <a href="https://github.com/Netflix/RxJava/wiki/Observable#wiki-onnext-oncompleted-and-onerror">RxJava Wiki: onNext, onCompleted, and onError</a>
+     * */
+    public final void forEach(final Action1<? super T> onNext) {
+        subscribe(onNext);
+    }
+    
+    /**
+     * Subscribes to the {@link Observable} and receives notifications for each element and error events.
+     * <p>
+     * Alias to {@link #subscribe(Action1, Action1)}
+     * 
+     * @param onNext
+     *            {@link Action1} to execute for each item.
+     * @param onError
+     *            {@link Action1} to execute when an error is emitted.
+     * @throws IllegalArgumentException
+     *             if {@code onNext} is null
+     * @throws IllegalArgumentException
+     *             if {@code onError} is null
+     * @throws IllegalArgumentException
+     *             if {@code onComplete} is null
+     * @see <a href="https://github.com/Netflix/RxJava/wiki/Observable#wiki-onnext-oncompleted-and-onerror">RxJava Wiki: onNext, onCompleted, and onError</a>
+     * */
+    public final void forEach(final Action1<? super T> onNext, final Action1<Throwable> onError) {
+        subscribe(onNext, onError);
+    }
+    
+    /**
+     * Subscribes to the {@link Observable} and receives notifications for each element and the terminal events.
+     * <p>
+     * Alias to {@link #subscribe(Action1, Action1, Action0)}
+     * 
+     * @param onNext
+     *            {@link Action1} to execute for each item.
+     * @param onError
+     *            {@link Action1} to execute when an error is emitted.
+     * @param onComplete
+     *            {@link Action0} to execute when completion is signalled.
+     * @throws IllegalArgumentException
+     *             if {@code onNext} is null
+     * @throws IllegalArgumentException
+     *             if {@code onError} is null
+     * @throws IllegalArgumentException
+     *             if {@code onComplete} is null
+     * @see <a href="https://github.com/Netflix/RxJava/wiki/Observable#wiki-onnext-oncompleted-and-onerror">RxJava Wiki: onNext, onCompleted, and onError</a>
+     * */
+    public final void forEach(final Action1<? super T> onNext, final Action1<Throwable> onError, final Action0 onComplete) {
+        subscribe(onNext, onError, onComplete);
+    }
+    
     /**
      * Groups the items emitted by an Observable according to a specified criterion, and emits these grouped
      * items as {@link GroupedObservable}s, one {@code GroupedObservable} per group.