[TwigBundle] Fixed escaping of service identifiers in configuration

To recreate, add the following to `config.yml`:

    twig:
        globals:
            foo: "@@bar"

**Expected behaviour:** Twig has a global variable named `foo` containing the string `@bar`.

**Actual behaviour:** ServiceNotFoundException: The service "twig" has a dependency on a non-existent service "@bar".

Environment tested on:
- Symfony Standard Distribution 2.3.2
- OS X 10.8.4
- PHP 5.4.16