Add in the -g (group / gid) switch

Hi,

Thanks for this very useful tool.

I tried it on my Raspbian to start XBMC on WOL and encountered the following errors:
`* failed to open vchiq instance`

Googling reveals that XBMC has to be started with the gid of the `video` group, but my account (`pi`)'s primary gid is not `video`. So I modified your program a bit to accept a user-specified gid for forking the process. After this, I'm able to start my Raspbian's XBMC on WOL.

Just FYI, you should ignore the `README.md` changes, and handle this pull request as you see fit.