Fix eZURLWildcard: wildcardsIndex is never cached

At the moment, the eZUrlWildcards Index is never cached in the class and so fetched from the cache file or recreated on each call.

This patch ensures that the wildcards index is cached in class on first call and stored until `expireCache()` is called. In addition, the PHPDocs of the `wildcardsIndex()` and `createWildcardsIndex()` are matching the method's params and return values.

Cheers
:octocat: Jérôme