Add support for GET /merge_requests call #553

Description of the problem, including code/CLI snippet

Since GitLab 9.5 there is a GET /merge_requests call that returns all the merge requests the current authenticated user have access to.

This call is very useful when doing actions over all the merge requests, as is much more efficient than doing GET /projects/:id/merge_requests for each project.

A good place for its manager to be would be in the mergerequests property in the client object, the same name used in project objects.

• python-gitlab version: 1.5.1
• API version you are using (v3/v4): v4
• Gitlab server version (or gitlab.com): 9.5 and greater.
• API call URL doc: https://docs.gitlab.com/ee/api/merge_requests.html#list-merge-requests