Added subscriber options to HttpCache

Based on #177, ref #170

This PR adds 2 options that one can add in their `getOptions()`:

* `fos_native_subscribers`: bit options to activate/deactivate subscribers provided by FOSHttpCacheBundle
  > By default **all native subscribers are activated** (`HttpCache::SUBSCRIBER_ALL`).

* `fos_subscribers`: Array of custom subscribers to add to the event dispatcher

## Examples with `fos_native_subscribers`
**ALL native subscribers EXCEPT the user context one**
```php
protected function getOptions()
{
    return [
        'fos_native_subscribers' => self::SUBSCRIBER_ALL | ~self::SUBSCRIBER_USER_CONTEXT
    ]
}
```

**NO native subscribers**
```php
protected function getOptions()
{
    return [
        'fos_native_subscribers' => self::SUBSCRIBER_NONE
    ]
}
```

**ONLY user context subscribers**
```php
protected function getOptions()
{
    return [
        'fos_native_subscribers' => self::SUBSCRIBER_USER_CONTEXT
    ]
}
```

**User context subscriber AND Foo subscriber**
```php
protected function getOptions()
{
    return [
        'fos_native_subscribers' => self::SUBSCRIBER_USER_CONTEXT | self::SUBSCRIBER_FOO
    ]
}
```

## Examples with `fos_subscribers`
**NO native subscribers + custom subscriber **
```php
protected function getOptions()
{
    return [
        'fos_native_subscribers' => self::SUBSCRIBER_NONE,
        'fos_subscribers' => [new MySubscriber()]
    ]
}
```

**ALL native subscribers (default) + custom subscriber **
```php
protected function getOptions()
{
    return [
        'fos_subscribers' => [new MySubscriber()]
    ]
}
```
i would prefer to remove this option and only keep the bitmap.

creating those subscribers should be done outside the kernel, or can be done in the constructor. there is no benefit in the indirection that i can see.