Add opentok.close() method. Resolves #121

Author: aiham

README.md

@@ -62,6 +62,12 @@ String apiSecret = "YOUR API SECRET";
 OpenTok opentok = new OpenTok(apiKey, apiSecret)
 ```
 
+And make sure you call `close` when you are done to prevent leaked file descriptors.
+
+```java
+opentok.close();
+```
+
 ## Creating Sessions
 
 To create an OpenTok Session, use the `OpenTok` instance's `createSession(SessionProperties properties)`

src/main/java/com/opentok/OpenTok.java

@@ -451,5 +451,9 @@ public OpenTok build() {
 
             return new OpenTok(this.apiKey, this.apiSecret, clientBuilder.build());
         }
-  }
+    }
+
+    public void close() {
+        this.client.close();
+    }
 }