Add HttpCache client with Filesystem cache.

By this PR, I'm introducing a new HttpClient with caching features.
Cache content is always fresh because it use the If-Modified-Since header. If the cached url is fresh enough, only response headers are transferred from thetvdb.com and content is loader from cache.

The CacheClient also have a ttl parameter which tells how many seconds requests on cached url are not sent to server and content directly loaded from cache. To disable this behavior, just set the ttl to -1.

Usage :
```php
$cache = new Moinax\TvDb\Http\Cache\FilesystemCache(__DIR__ . '/cache');
$client = new Moinax\TvDb\Http\CacheClient($cache, 600);
$tvdb = new Moinax\TvDb\Client("http://thetvdb.com", "YOURAPIKEY");
$tvdb->setHttpClient($client);
```