add serialize method.

Author: webee

README.md

@@ -15,6 +15,6 @@ To install the library add:
         maven { url "https://jitpack.io" }
    }
    dependencies {
-         compile 'com.github.webee:java-json-api:v2.1.0'
+         compile 'com.github.webee:java-json-api:v2.2.0'
    }
    ```

src/main/java/com/github/webee/json/JSON.java

@@ -10,6 +10,7 @@ public interface JSON {
     WritableJSONObject newObject(Map<String, Object> map);
     WritableJSONArray newArray();
     WritableJSONArray newArray(Object[] array);
+    String serialize(Object value);
     // parse to JSON domain types.
     Object parse(String text);
     JSONObject parseObject(String text);