#57 Update MetaModelSelect.php (state: open)

I added a cache into the getFilterOptions() and the getFilterOptionsForDcGeneral method. Maybe it is not perfect but it solved an performance issue into my project, where the size of options counts 4000 items.

While I agree that a caching solution might come in handy, I am not happy with the current implementation in this PR. The cache invalidation is way too cumbersome and hard coded, we should agree upon a generic cache layer. This also means a new feature and will come with 2.1.0 earliest.

See also: #58, MetaModels/attribute_tags#57, MetaModels/attribute_tags#58

ok, how ever. Maybe someone want to use it as fix till the feature in implemented :) Thx for feedback!!