Cancellable is returned by public functions, but isn't accessible (#157)

* Cancellable is returned by public functions, but isn't accessible

* add cancel into exampl on README and fix fromJson example to use Factory
diff --git a/README.md b/README.md
index 22230c6..a020123 100644
--- a/README.md
+++ b/README.md
@@ -68,10 +68,13 @@
 ```java
  LottieAnimationView animationView = (LottieAnimationView) findViewById(R.id.animation_view);
  ...
- LottieComposition composition = LottieComposition.fromJson(getResources(), jsonObject, (composition) -> {
+ Cancellable compositionCancellable = LottieComposition.Factory.fromJson(getResources(), jsonObject, (composition) -> {
      animationView.setComposition(composition);
      animationView.playAnimation();
  });
+
+ // Cancel to stop asynchronous loading of composition
+ // compositionCancellable.cancel();
 ```
 
 You can then control the animation or add listeners:
diff --git a/lottie/src/main/java/com/airbnb/lottie/Cancellable.java b/lottie/src/main/java/com/airbnb/lottie/Cancellable.java
index 6eeac5e..b71470e 100644
--- a/lottie/src/main/java/com/airbnb/lottie/Cancellable.java
+++ b/lottie/src/main/java/com/airbnb/lottie/Cancellable.java
@@ -1,5 +1,5 @@
 package com.airbnb.lottie;
 
-interface Cancellable {
+public interface Cancellable {
   void cancel();
 }