Note that this is an API-breaking change but should have minimal
impact on users of this package due to the nice inference
properties of the Go programming language.

Bumped `libraryVersion` to `2` due to API-breaking change as
discussed in #376.

Fixes #180.

Change-Id: Ib386135e6b8f306d1f54278968c576f3ceccc4e7

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.