Add Subscription.and(other) method.

For Subscription composition.

reactfx/src/main/java/org/reactfx/Subscription.java

@@ -4,8 +4,23 @@
 public interface Subscription {
     void unsubscribe();
 
+    /**
+     * Returns a new aggregate subscription whose {@link #unsubscribe()}
+     * method calls {@code unsubscribe()} on both this subscription and
+     * {@code other} subscription.
+     * @param other
+     */
+    default Subscription and(Subscription other) {
+        return new BiSubscription(this, other);
+    }
+
     static final Subscription EMPTY = () -> {};
 
+    /**
+     * Returns a new aggregate subscription whose {@link #unsubscribe()}
+     * method calls {@code unsubscribe()} on all arguments to this method.
+     * @param subs
+     */
     static Subscription multi(Subscription... subs) {
         switch(subs.length) {
             case 0: return EMPTY;