Added a rewritten host and hostgroup section and started a service section.
Added line about installing Apache and Postfix so there are network-reachable services that Icinga can check; started section about email notifications.
Merge branch 'nrpe_section_refactor': This branch turned out to contain a lot more than just a refactor of the NRPE section, so I'm merging it back in and will create topic branches later for smaller pieces of it as I go.