Support for FreeBSD and few other features

Summary of the changes:

  - "${apache::params::conf_dir}/Includes" as $apache::confd_dir for FreeBSD,
  - "${apache::params::conf_dir}/Modules" as $apache::mod_dir FreeBSD,
  - "${apache::params::conf_dir}/Vhosts" as $apache::vhost_dir FreeBSD,
  - added to apache::params: $root_group, $apache_package, $server_root,
    $mime_support_package, $mime_types_config
  - ServerRoot now uses $server_root instead if $httpd_dir,
  - added mpms: event, peruser, itk (PR #304 + FreeBSD support)
  - added $mime_support_package parameter to apache::mod::mime class,
  - apache::mod::mime installs $mime_support_package package if needed,
  - added $magic_file parameter to apache::mod::mime_magic class,
  - added 'default_confd_files.pp', and confd template infrastructure to
    allow putting some files under conf.d/ (Includes/ under FreeBSD) by
    default (FreeBSD's apache22 installs Includes/no-accf.conf for
    example but puppet normally would purge it afterward),
  - touched documentation (README.md),
  - other (minor or forgotten) changes and additions,