[Server] implement check another server process running

This implementation for daemonized hatohol server process. #40 

* add `checkAnotherServerProcess()` function to check another hatohol server process running
* `checkAnotherServerProcess()` checks existence of hatohol's pidfile.
* When another daemonized hatohol server process running, it will be automatically exit.

Does it need implementation for given `--foreground` option case?