Configure cache TTL in a consistent, managable way

Introduce standard caching times:

```
	const CACHE_LONG = 2592000; // 30 days
	const CACHE_STANDARD = 86400; // 24 hours
	const CACHE_SHORT = 10800; // 3 hours
```

Introduce ``WikiaResponse::setCachePolicy`` for forcing private caching policy.

@michalroszka / @prein