API: Filter issues by labels

While working on [laboard](https://gitlab.com/jubianchi/laboard) I had some issues with the API : I was not able to filter issues by labels so I had to write some logic to do it on my side.

I think it will be useful to have this feature supported by the Gitlab API.

I added the filtering logic on `/issues` and `/projects/:id/issues`.

The commits are not squashed but I'll do it once everything seems ok to you.

See merge request !169