Add note about x-www-form-urlencoded not being used
Update JSON parsing callback API, add JSON tests.

- JsonCallback#onSuccess() no longer throws a JSONException.
- JsonUtil#fromJson() no longer throws a JSONException.
- JsonCallback now can parse all items, not just arrays and objects.
- JsonObjectCallback and JsonArrayCallback now extend JsonCallback.
- Added Json parsing unit tests.
- Bump version number.