Add support for Debian

Cherry-picked ce53b8f8e2ba5c7dd23755efc38fb460af7fc25b from Martijn de Gouw's
(https://github.com/martijndegouw) fork for Debian support.
Packages to be installed can now be specified

Added the packages parameter to init.pp. It can be used to override which
packages this module installs.
Added manage_cron parameter.

In response to reading https://github.com/unbc/puppet-sssd/issues/16, I've added
a new parameter that will control whether or not cron restarts when sssd does.
Limited homedir.pp class to RedHat machines

Until further testing can be done, the make_home_dir parameter will only be
respected by RedHat machines.
Updated spec tests

Added appropriate spec test sections for Debian machines.
Updated documentation
Merge branch 'add_debian_support' into develop

- Added basic support for Debian hosts
- Added packages paramater to allow users to override what packages this
  module installs
- Added manage_cron parameter to control whether or not cron restarts when
  sssd does
- Updated spec tests & documentation