Support for FreeBSD and few other features

Summary of the changes:

Overview:

* added support for FreeBSD
* added MPMs: event, peruser, itk (PR #304 + FreeBSD support)
* added `apache::package` to choose and install apache package,
* allow apache::mod to specify mod identifier and module path (#271)
* revisited specs for apache::dev and apache::mod::dev

Details:

* "${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,
        $service_name,
        $server_root,
        $mime_support_package,
        $mime_types_config
* httpd.erb now uses $server_root (instead of $httpd_dir) as ServerRoot
* 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),
* adjusted documentation (README.md),
* apache::dev requires apache::package on FreeBSD
* other (minor or forgotten) changes and additions,