Paginate terms queries and limit by default #534

#347 was just fixed, providing the ability to filter the collection retrieved by /taxonomies/<taxonomy>/terms endpoints. With this, we now have the ability to set the number filter value to specify the number of terms to retrieve.

When the number of terms is limited, we should provide pagination headers for terms collections in the same manner as we do with query_navigation_headers for posts collections.

Once pagination headers are available we should also limit the number of terms retrieved by default, in a manner similar to the way /posts defaults to the number of posts per page specified in settings. Without an imposed limit, unfiltered posts queries on sites with tons of tags, such as http://www.wired.com/wp-json/taxonomies/post_tag/terms, will become so large that they timeout fail when there are too many terms available.