Initial RPM packaging & instructions

Hi,

This is initial RPM packaging work for Ghost. It's tested on CentOS 6.x, but should be considered beta. Upgrade between releases works smoothly.   The packages is build to take on the work in  TryGhost/Ghost#1364 , TryGhost/Ghost#1100  (/etc/ghost.conf exists as a symlink for now)

I'd be happy to work with the team to set up an official YUM repository for Ghost, with proper package signing, if that would be of use?

Matt