add params to listCommits

To allow use of the "ref_name" parameter, *listCommits* should pass the *parms* object so that we're able to get commits from a specific branch/tag. 

e.g.
```
gitlab.projects.listCommits({id:1,ref_name:"v1.0"},function(commits){  });
```

From the spec:
https://github.com/gitlabhq/gitlabhq/blob/master/doc/api/commits.md