[POC] Content negotiation implementation.

This PR attempts to implement server content negotiation features to Symfony2 components, as described here: http://httpd.apache.org/docs/2.2/en/content-negotiation.html.

**How should it work:**

1. Let says I wish to let the framework negotiate the format & locale, I would write my routes like this:

        my_route:
            pattern: /my/route.{_locale}.{_format}
            requirements:
                _locale: en|es|fr
                _format: html|json|xml
            options:
                negotiate: [_locale, _format]

2. I need to have the following headers in my request:
    *  `Accept` which gives the accepted content types (`_format`)
    *  `Accept-Language` which gives the accepted languages (`_locale`)

3. I want to get the best resource variant when requesting `/my/route` URL.

**The following rules should do the trick:**

*  If none of a required parameter (ie. `en`, `es` or `fr` for `_locale`) is present the corresponding `Accept` header (ie. `Accepted-Language` for `_locale`), a 406 exception is thrown.
*  Else the value with the highest quality is selected and passed to route parameter.

Here is an example: https://gist.github.com/3872155