New format of handling user.absent, supporting additional parameters

My use case was to remove a user **including her home directory**. The original users-formula did not provide support for additional parameters of "user.absent" ("purge" and "force"). Removing a homedir can obviously be achieved some other way, however I wanted this to happen automagically.

This patch adds support for three additional pillar values: "absent", "purge" and "force". The main *for* loop excludes users in which "absent" in present (pun intended :) and does the usual work, while the additional *for* loop goes only through the ones with "absent" and makes use of parameters.

I've provided new examples in the pillar.example file:

    users:
      jenny:
        absent: True
        purge: True
        force: True

I also left support for "absent_users" syntax for backwards compatibility:

    absent_users:
      - donald
      - bad_guy

I'm hoping you will find this modification useful. Thanks.