Add `XQueryKeymap` command

[James-Livesey]

This PR follows up on https://github.com/sidorares/node-x11/issues/69#issuecomment-54241340 by adding a function call for the XQueryKeymap command.

This enables global tracking of key presses (independent of window focus) that makes implementing applications relating to keylogging possible, such as for the functionality seen with the screenkey application.

This command will be used in the gShell desktop environment to bind global shortcuts.