Extending Runners API to include GET /runners call

The purpose of this pull request is for the automation of setting up a new project with a runner.  

The `GET /runners` call retrieves a list of all runners registered on the Gitlab CI instance.

Currently, there is no way of finding the ID of an existing runner using the API.  Since all projects need to be linked to a runner with a known ID, this pull request provides a way to look up that information.

The following can now be utilized in a "hands-free" way:

    POST /projects/:id/runners/:runner_id
    DELETE /projects/:id/runners/:runner_id

Thanks again and looking forward to your thoughts.

@randx @dosire