update package.xml
add mutable method.

Currently, get method allocate new message when specified child message
does not exist. it's easy to use some cases.
mutable method will allocate and set child message when specified field
does not exist.

```
$person = new Person();
$phone = $person->getPhone(); // you have to set $phone manually.
$phone = $person->mutablePhone(); // allocate and set phone message.
$phone->setNumber(12345);
```