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
public function getOptions()
{
    return [
        'fos_native_subscribers' => self::SUBSCRIBER_ALL | ~self::SUBSCRIBER_USER_CONTEXT
    ]
}
```

**NO native subscribers**
```php
public function getOptions()
{
    return [
        'fos_native_subscribers' => self::SUBSCRIBER_NONE
    ]
}
```

**ONLY user context subscribers**
```php
public function getOptions()
{
    return [
        'fos_native_subscribers' => self::SUBSCRIBER_USER_CONTEXT
    ]
}
```

**User context subscriber AND Foo subscriber**
```php
public function getOptions()
{
    return [
        'fos_native_subscribers' => self::SUBSCRIBER_USER_CONTEXT | self::SUBSCRIBER_FOO
    ]
}
```

## Examples with `fos_subscribers`
**NO native subscribers + custom subscriber **
```php
public function getOptions()
{
    return [
        'fos_native_subscribers' => self::SUBSCRIBER_NONE,
        'fos_subscribers' => [new MySubscriber()]
    ]
}
```

**ALL native subscribers (default) + custom subscriber **
```php
public function getOptions()
{
    return [
        'fos_subscribers' => [new MySubscriber()]
    ]
}
```
i expected you would add a bitmask to the constructor, to allow to configure which subscribers to autoload. then an extending user could just replace that bitmask. and he also could create his custom subscribers in his custom constructor. not sure this whole options mechanism is that useful, especially for registering custom classes. its just more complicated without any extra flexibility.
for the subscriber bitmap, we can discuss getOptions as the bitmask indeed is some sort of option. as we configure things in the constructor, we do not need it however.