RHEL Support

Hi, this is a revival of @snobear's PR #9 for RHEL5 & 6 (sysvinit) support.

It adds support for systemd, but only triggers on Fedora and Centos/RHEL 7+.  Centos 7 is fine but I don't have a 6 server to test on. The init scripts are mostly the same from the previous PR so any issues should be minor and you can assign anything raised to me.

I've tried not to change anything in the Ubuntu configs except for reworking the tests.
I'll update the doc.  I'd intended to do like-for-like (apache is equivalent), but it then places a dependency on Apache.  I'd prefer to allow it to install without dependencies, and allow user to change if they're using Apache/Nginx.