added a nuget tool class

Added a nuget tool class that retrieves the nuget executable

Requires zopaUK/bozo#28
I'm uneasy about this for a couple of reasons: 

* Should we be hot linking to a specific version of nuget (which I think we've upgraded from already!)
* Is there a more secure way of getting the exe to the machine?

Perhaps we should consider hosting a version somewhere, perhaps the nuget package server itself?