[Parser] Added Configuration to globally enable or disable parsers

Hey There,

because of the Problems with #295 and #291 i have implemented the Feature to disable Parsers. It  is possible to deactivate the Parsers per Action but that is a pain in the ass if you want to disable them on every action. So if this is merged you can disable them in the config like:

``` yml
# app/config.yml
nelmio_api_doc:
    parsers:
        ValidationParser: false
```

All existing Parsers (at state of this PR) are enabled by default
It makes sense if you want to check if the configuration is working with the Service. So you need to get the registered Parsers to compare with the Configuration.