Update to start/stop script's use of 'su' command.

Hi,

After building OrientDB from source on my Mac (Mavericks), I encountered a problem starting it using orientdb.sh.

It boils down to the syntax of the 'su' command used to start and stop the server.

The previous syntax of:
su -c 'COMMAND' - USER

Is not valid on Macs and produces an error:
su: illegal option -- c

The correct syntax for the same command on Macs is:
su USER -c 'COMMAND'

This seems to work on Linux as well, I tested the command syntax on Fedora 20.

I mentioned this on Twitter and Luca requested a pull request so, here it is. :)

Best,

Jeremiah Hall