updated server.py to create dir if not available

fix #2 
> I do config.sqlite_db because the user might want to store this in a directory other than centinel_home.

but they can't -- https://github.com/iclab/centinel-server/blob/master/config.py#L16

if they change it to a diff directory then that's their problem, they should only change the filename

> As to your second point, are you suggesting that I create centinel_home if it doesn't exist or that I move this check outside the other check?

yes, the db can be created only inside centinel_home