IssueRequest struct for create/edit. Fixes #75.

Using an *Issue in IssuesService.{Create,Edit} means that the Labels
and Assignee fields do not serialize to the correct JSON representation
for the GitHub API. This change introduces an IssueRequest struct to
use instead, allowing labels and assignees to be set on create or edit.
add Alex Bramley as contributor

(Alex is a Googler, so no change to AUTHORS file)