Add a macro which gives the number nodes in a category or locale.

@NODE_COUNT [[Category Pubs]] will return the number nodes in the Pubs category.
This fixes #20
Dont need to sort if you're just counting.

Also refactor so that we just pass a number not an array.
Merge pull request #70 from OpenGuides/20_node_count_macro

Add a macro which gives the number nodes in a category or locale.