cool-cats: only fetch the ids to limit footprint

We don't really care much about the returned; all we want is the count
so there's no point in fetching all the category data.
cool-cats: Limit number of categories fetched

We only need to know if there is more than one category so only fetch a
maximum of two categories. The rest of the categories are all gravy.
Merge pull request #445 from Automattic/speedy-cool-cats

_s: Optimize `_s_categorized_blog()`.