Add ability to change worker configuration

This patch adds the ability to have a custom mpm_worker configuration.
Updating apache::mod::prefork and adding apache::mod::worker

Also adding debian support
Adding mpm_module parameter to the base class with redhat/debian support
Adding specs for mpm worker/prefork
Update docs for mpm_module and prefork/worker mod
Fix for mpm_module validation
Merge pull request #38 from hunner/worker

Support RedHat/Debian mpm prefork & worker modules