Fixes #244

[docs] Add docs to private functions; fix typo.

The RateLimit() func is deprecated, and should not appear in the
official example.

As of [fccbb9520b (diff-e754c06a3071f63f53cee61571452b4c),) `Users.List` method was updated to `Users.ListAll`. This commit updates the documentation for `UserListOptions` to reflect that change.

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.

This change adds the fields "DiffHunk", "OriginalPosition", and "OriginalCommitId" to the PullRequestComment struct. These fields provide important information about the location of a comment.

Fixed documentation from Issue 16

Fixed GoDoc badge.

Fixes #119

Reduce variable name restriction