Handle PowerShell parameters passed via splatting

Allows PowerShell arguments such as the following to be passed to scripts/cmdlets:

    script: myscript.ps1 "@{This='THIS'; That='THAT'; Other='OTHER'}"

More info on splatting: http://technet.microsoft.com/en-us/magazine/gg675931.aspx

Via discussion on ansible-project mailing list: https://groups.google.com/forum/#!topic/ansible-project/jojvFohOwqM