Updated for CentOS 7 build and install

- .spec file was reused from Fedora and updated ::
  - creates system user + group gmediarender upon installation and add to group 'audio', also removes all on package uninstalltion, homedir is set to /usr/share/gmediarender/ so ownership is also updated
- .service file was reused from Fedora and updated ::
  - service runs as unpriviledged user 'gmediarender'