[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
The System is working with Tags. To be compatible with existing Parsers there is no other way cause the method that searches for tags does not have an excluding list.