Make it possible for callers of IssuesService.ListMilestones() and
similar to step through pages of results.

Additional fields (html_url, labels_url, id, closed_at) pulled from
https://developer.github.com/v3/issues/milestones/

Fixes #295

API URLs starting with "/" does not play well with Github Enterprise,
whose API endpoint looks like "http(s)://hostname/api/v3/".

This fixes incorrect URLs in miletones APIs.