Gracefully handle multiple module versions

[briantist]

I recently discovered that Invoke-DscResource fails when there is more than version of a module installed. You can explicitly give it a version of the module, but it won't discover it for you.

So the proposal here is two-fold:

1. Ensure Idempotion can handle a module specification [hashtable] (not just a name).
2. Default behavior when given only a name should be to discover local versions and use the latest automatically.