Adding 'stopbypid' command for forever

As requested in [issue 492](https://github.com/nodejitsu/forever/issues/492). 

I've decided to add 'stopbypid' command to **forever** for convenience. 
Even though name is not as good as i want it to be...it is still ok (i think). 

Example of this command usage:
```bash
forever stopbypid 1239
```