Unified and configurable front controller

Having to change the `.htaccess` and/or edit `web/app.php` and `web/app_dev.php` depending on the environment can tedious, cause discrepancies between branches in Git or force to use .gitignore files.

Since Symfony 2.3, `parameters.yml` is ignored and generated via `parameters.yml.dist`, so I had the idea to use the same concept to configure the Kernel loading. Parameters are written in INI because Yaml is too slow for the critical needs of a front controller.

Used parameters are:

* `environment`: (dev|test|prod)
* `debug`: (true|false) guessed automatically on environment == dev
* `http_cache`: (true|false) whether or not to use Symfony HTTP reverse proxy
* `localhost_only`: (true|false) whether or not to limit request to localhost when in dev mode
* `umask_fix`: (true|false) whether or not to change the umask to 0000
* `apc_cache_id`: (false|string) if a valid string, use this as an APC id for the caches

`web/app.php` is now generic, it loads those parameters and its behaviour depend on them.

Front controllers (ex: app_dev.php) can override the options and developers can override the `AppLoader` for custom code or additional parameters.

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Other pertinent point: Seeing PHP code in a front controller can be confusing. In my solution, the PHP code is abstracted away from the front controllers, only parameters are exposed. IMO, this is easier to edit and document.

I must say, the first time I saw this chunk, I immediately thought: “I have no idea what voodoo this is doing, but I know I must not touch it”. Why not hide it somewhere deeper ?
```php
<?php
Request::enableHttpMethodParameterOverride();
$request = Request::createFromGlobals();
$response = $this->kernel->handle($request);
$response->send();
$this->kernel->terminate($request, $response);
```

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For the code of the loader itself and complete documentation, see my repo https://github.com/wemakecustom/symfony-app-loader. It was developed as a separate project, but if this PR goes through, it can be moved to the Symfony namespace, as long as some credit remains.

Of course, the concept is open for discussion; if someone can find a better way to do something similar, I would be glad to hear it.

_NB: app/console also uses the `environment` and `umask_fix` settings._