Add author object to commits API endpoint

Problem to solve

GET /projects/:id/repository/commits lacks detailed author information. https://docs.gitlab.com/ee/api/commits.html#list-repository-commits

Further details

Github and Bitbucket include detailed author and commiter information in their List commits on a repository endpoint: https://developer.github.com/v3/repos/commits/.

Proposal

Include the author object in GET /projects/:id/repository/commits

"author" : {
      "web_url" : "https://gitlab.example.com/thedude",
      "avatar_url" : "https://gitlab.example.com/uploads/user/avatar/28/The-Big-Lebowski-400-400.png",
      "username" : "thedude",
      "state" : "active",
      "name" : "Jeff Lebowski",
      "id" : 28
   },