Parameter generator backslash escaping

Found while trying different default parameters for methods. Currently, `Zend\Code` is not able to produce valid code when following is being done:

```php
$generator = new Zend\Code\Generator\ParameterGenerator();

$generator->setType('Foo');
$generator->setDefaultValue('\\');
$generator->setName('bar');

var_dump($generator->generator());
```

Produces

```php
Foo $bar = '\'
```

Which is obviously going to break method signature if used as a method parameter.