added $apt_disable_update to disable "apt-get update" during puppetruns
updated README to explain $apt_disable_update
comparing variabled should use double quotes (interestingly, it does work with only one too)
switch default $debian_url to use http.debian.net which is more accurate and up-to-date than cdn.debian.net
On a sid system, $lsbdistcodename is reports 'n/a', this doesn't work because the module
looks for a template based on the $lsbdistcodename, so you get this error:

      Could not find template 'apt/Debian/preferences_n/a.erb'

The slash in 'n/a' appears to be a path (confusing!)

So I've set an error message when 'n/a' is encountered, encouraging the user to set the $lsbdistcodename.

I was not confident in the idea of pointing any occurrence of 'n/a' to the sid
templates, as I could imagine a case where this might occur on a non-sid system
where the lsbdistcodename isn't properly detected
exec{'apt_dist-upgrade'} just requires Exec['apt_updated'] if apt::disable_update if false
Merge remote-tracking branch 'riseup/feature/unknown_lsbdistcodename'
Merge remote-tracking branch 'riseup/feature/http_debian_net'