total_count support

total_count is sent by Redmine to all of the lists and is really useful especially when "paginating" results. 

One issue might be the fact that with lazy-evaluating Resources / ResourceSets, total_count will not be available until AFTER evaluation, so forcing evaluation might be necessary (e.g calling list(issue_list)). However, total_count is especially useful when working with a subset (limit, offset) of resources, so evaluating the whole set might not be such a big problem.