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
},
```
issue