Support repository settings to configure the allowed PR merge methods

[ryangribble]

As per the recent announcement the repository settings to enable/disable the 3 Pull Request merge options (Merge, Squash, Rebase) are now configurable via the API 🙌

There are 3 new properties to accomplish this, which will need to be added to our Repository response class and also the Create and Edit requests will be updated with new request parameters

API Docs
- Create (this turned out to be a documentation error, and the Create method does not support these new fields at this time)

{
  "name": "Hello-World",
  "allow_rebase_merge": false,
  "allow_squash_merge": true,
  "allow_merge_commit": false
}

• Get

{
  "id": 2278524,
  "name": "hubot",
  "full_name": "github/hubot",

  "allow_rebase_merge": true,
  "allow_squash_merge": true,
  "allow_merge_commit": true,

}

• Edit

{
  "name": "Hello-World",
  "allow_rebase_merge": false,
  "allow_squash_merge": true,
  "allow_merge_commit": false
}