Support for testing is server running and for stopping servers

[pekkaklarck]

There has been some needs for both testing is a server running and easily stopping a running server. It would be convenient if the remote server itself supported them like this:

python -m robotremoteserver test
python -m robotremoteserver stop lolcathost:12345

This can be implemented by connecting to the remote server via XML-RPC and using stop_remote_server method for stopping it (if allowed) and get_keyword_names to test is it running. This approach supports servers running on remote machines and also other remote server implementations.

[pekkaklarck]

The above commit also resolved this issue.