Add support for Contributors

[creamidea]

Add support to call the API-Functions of contributors https://docs.gitlab.com/ce/api/repositories.html#contributors for next release.

[L3au]

+1

[L3au]

@creamidea You can implement it temporarily with you own

const Repository = require('node-gitlab/lib/resources/repository')

Repository.prototype.getContributors = function (params, callback) {
  params.type = 'contributors';
  this.list(params, callback);
};