Apt update tooling

This gives you the ability to have puppet run apt-get update a few different ways:

  * always
  * daily
  * weekly
  * reluctantly **default**

it determines when to trigger the apt-get exec via a combination of an addition of a script in apt.conf.d that touches a file when apt-get update runs, and a  custom fact which checks the mtime of that file.

all behavior should be covered by spec tests.

additionally fixed up the spec tests to function when running natively on non-debian systems

Please let me know if there are any changes you'd like to see here in order to merge this
You should not `include apt` here. It is fair to assume that anyone using `apt::update` will have already included apt with any necessary parameters before they include this class.