Add a config option for a default Accept header

I'm not sure if this is ideal for everyone, but I thought I'd submit the PR anyway. 

If an API looks at the HTTP Accept header when deciding what format to return we need to manually enter a header into the sandbox's header field, i.e. application/json, application/xml, etc, this can get a little cumbersome/boring when playing around with multiple services in the sandbox. This PR adds the option to set a default in the config as follows:
```
# app/config.yml
nelmio_api_doc:
    name: My API
    sandbox:
        accept_type: application/json
```

You will then by default have that header value already populated in the sandbox field.

![PR](https://f.cloud.github.com/assets/727603/7320/b5d2393e-4400-11e2-9129-9a0f2303c8a9.png)