Taxonomy Scaffold: Use gettext context

Per instructions in the Codex, use gettext context when specifying a `singular_name` label.

> Default is _x( 'Post Tags', 'taxonomy general name' ) or _x( 'Categories', 'taxonomy general name' ). When internationalizing this string, please use a gettext context matching your post type. Example: _x('Writers', 'taxonomy general name');

See http://codex.wordpress.org/Function_Reference/register_taxonomy#Arguments