Add default Windows support

Previously, the hard-coded default staging path of "/opt/staging" precluded touch-free use of this module on the Windows platform. This commit adds the params pattern in order to allow the module to work on Windows platforms without modification.

Further development would be a nice-to-have in terms of adding support for downloading various kinds of URLs; this is just the critical fix.

In the future, the params pattern may be removed in favor of Data-in-Modules, but this functionality is not yet well developed in Puppet. Puppet v3.4 should introduce good support for such things, but this is the best way I know how to do it until then.