Changed powershell exec resources and some refactoring

Hi Peter,
This patch changes the exec resources that call powershell to use the powershell provider.

Also, some refactoring is done:
 * I saw that your last commit had some conflicts, I solved them.
 * Changed init.pp to include the commands class as it was before.
 * Changed commands.pp to be defined again as a class.
 * With the powershell provider, we don't need the winpaths definition in init.pp, it's cleaner now.
 * A few lines for class documentation in init.pp

Ummm... I think these are the main points...

Take a look and see if it's ok to merge :)