Adding a new powershell wrapper

When working with windows there are many times when you will want to execute powershell commands or scripts. 

This is not always as easy as it might seem and there are issues such as passing the correct parameters to make sure it's run silently and making sure that your using the correct powershell.exe (if it's not in your path).

Most of these edge cases have already been covered in [puppetlabs-powershell](https://github.com/puppetlabs/puppetlabs-powershell/blob/master/lib/puppet/provider/exec/powershell.rb).

It would be useful to have a wrapper that deals with all this makes it simple to execute powershell commands on a host.

Something like:
 `on host, powershell('command to run here')`