Rename 'gitlab-git-http-server' to 'gitlab-workhorse'

The name 'gitlab-git-http-server' no longer describes what the daemon does; it handles archive downloads too now, and we will handle general file uploads and downloads to its list of tasks.

@marin came up with 'workhorse'. I think this is a great name because it is not a bad description of what this project does, and it also makes fun of Unicorn.

What do you think @sytses @dzaporozhets ?