Peruser event

This PR adds event and peruser MPMs. These are not available from any OSes currently supported by apache module. Working implementation may be found in  #342, which enables these MPMs on FreeBSD. I leave this PR in order to split #342 into smaller parts to make review process easier, see https://github.com/puppetlabs/puppetlabs-apache/pull/342#issuecomment-25423813.