pidfile messed up

The pidfile with the screen is being messed up with zombie screens, if the bukkit server is not instantanous "ready" and fails subsequently to start the whole thing up.

Solution:

``` Shell:
is_running() { 
...
if [ -r "$pidfile" ]
then
     screen -wipe >/dev/null     # add this line
```

This will clean up the messed pidfile.
