Add module_exists to system_service.module

This patch adds an interface to module_exists() for the system services module. You pass it the name of a module, and if the module is enabled on the remote system, the service returns the module's version. Otherwise it returns an empty string. This is useful for doing dependency checks between two servers, two verify that modules and versions match. I wrote this as part of my deployment project.

Let me know if you need anything!