Update and document operating system support

* Remove SUSE support (see below)
* Remove dead Gentoo code (Gentoo fails due to the logic in `package.pp`)
* Make operating system support consistent from Facter 1.6.2 (when osfamily fact added) through 1.7.4 (current)
  * Adds support for Ascendos, PSBM and XenServer when using older Facter versions
* Remove fallback to `$::operatingsystem` in `params.pp` (it's redundant since the module fails anyway if `$::osfamily` is undefined due to the logic in `package.pp`)
* Document supported operating systems
* Document minimum required Facter version

On SUSE: SUSE support hasn't had any functional updates since it was added in 2011. The current code installs `nginx-0.8` package, which applies only to OpenSUSE 11.4 (released March 2011) and won't work on any later versions since the package name changed. If users wanted up to date OpenSUSE support there would have been issues raised - given this isn't the case it seems safe to remove.

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If anyone searches the repo for "SUSE" later, find this and wants to add support, here are the package names required:

* On OpenSUSE 11.4 install package `nginx-0.8`
* On OpenSUSE 12.1 & 12.2 install package `nginx-1.0`
* On OpenSUSE 12.3 and later, install package `nginx`