Updates to run multiple instances of memcached.

Hi.  I have the same needs as discussed in https://github.com/saz/puppet-memcached/issues/18 so here's a PR to cover it.

I run rhel/centos servers so that is the only platform that I functionally tested.  I took a look at the startup scripts in the Debian package and it looks like this implementation should work, but its worth someone testing.  The difference as far as I can tell is that Debian will only have 1 service definition which manages all of the memcached instances whereas the other OS's have a service def for each instance.

For this implementation I removed the ability to specify the logfile, pidfile, and user.  It was necessary for the changes here and I don't feel like this will cause people major issues.  If you disagree perhaps we can come up with a middle ground solution.