Added subscriber options to HttpCache

Based on #177, ref #170

This PR adds 1 option 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`).

**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
    ]
}
```
Fixed fatal error due to namespace collision
Merge pull request #178 from lolautruche/subscriberOptions

Added subscriber options to HttpCache