allow clients to connect over unix domain sockets

[tdyas-stripe]

Allow debugger clients to connect to the in-process debug server over a UNIX domain socket instead of a local TCP socket. My company has been running a variant of this PR (against a 0.6.x release) internally since last September.

• start_server takes an additional socket_path argument. An error is raised if the host parameter is also provided at the same time. I'd love suggestions on how to improve the API.

• The PR refactors the tests enough to allow testing both the TCP socket and UNIX domain socket variants with the same test code.

[tdyas-stripe]

Looks like CI builds failed for Ruby v1.8.7 and v1.9.3. Any suggestions on how to modify the API to work on those earlier versions?

[tdyas]

This should be closed in favor of #183.

[alir2t2]

To connect client using a port I used this configuration shown below:

Launch.json

{
    "version": "0.2.0",
    "configurations": [
        {
            "name": "rdebug-ide",
            "type": "Ruby",
            "request": "attach",
            "remoteHost": "127.0.0.1",
            "remotePort": "1234",
            "remoteWorkspaceRoot": "${workspaceRoot}",
            "cwd": "${workspaceRoot}"
        }
    ]
}

rdebug-ide started with the following command:
rdebug-ide --port 1234 -- rails/server s

If I use unix socket instead of port, how do I connect my client to the unix socket?