[protocolbuffers\message] fixed #34 add jsonSerialize and toArray method.

now, protocolbuffers\message supported jsonSerializable interface.

```
$message = new SomeMessage(["id" => 1]);
echo json_encode($message);
```

also, add toArray method. this is useful when converting protobuf
message to an array.

Thease method has some limitations.

* JsonSerialize uses original field name (same as proto file) as hash key
* JsonSerialize might loose precision when using big number. this is PHP limitation.

please check this document.
https://github.com/chobie/php-protocolbuffers/blob/master/examples/Json.md