Implement "source" parameter

This adds a `source` parameter which can be set to one or more `puppet://`-style URLs. If set, it will read the first valid URL, parse it as an ini file, and create children `ini_setting` resources for each setting in the ini file that it just read.

This way, instead of specifying lots of `ini_setting` resources in the manifest, one can put a .ini file and generate a bunch of `ini_setting` rules from it.

I am pretty new to Ruby and have never written custom Puppet resources (or Puppet anything for that matter), so I'm sure there's some glaring mistakes or un-Ruby-ish code patterns in there, but at least it works from my admittedly limited testing.