Add reactions to commit comment payload

[shiftkey]

Link: https://developer.github.com/v3/repos/comments#preview-period-commits-comments

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 CommitComment response model
• include the preview header to methods in RepositoryCommentsClient which return commit comments: application/vnd.github.squirrel-girl-preview
• add tests to retrieve a known commit with reactions and verify it deserializes correctly