Add reactions to issue response payload

[shiftkey]

Link: https://developer.github.com/v3/issues/#preview-period-org-issues

Before we do this, let's get #1295 resolved as that will involve much of the groundwork.

After that, here's what I think needs to be done:

• add Reactions property to Issue response model
• include the preview header to methods in IssueClient which return issues: application/vnd.github.squirrel-girl-preview
• add tests to retrieve a known issue with reactions and verify it deserializes correctly

[martinscholz83]

@shiftkey, i take this issue and #1297 and #1298 if this ok. But I will wait of merge #1295 because of ReactionSummary

[ryangribble]

@maddin2016 are you still planning on doing #1296 #1297 and #1298 ?

[martinscholz83]

Oops 🙈 Of course!!! I didn't recognized that #1341 was merged. Thanks for the update