Pull down NuGet.exe as part of pre-build

[willduff]

Per @borrillis in pull request #41:

There needs to be a 'prebuild' step that ensures the nuget executable has been downloaded. Currently this is done in the build.cmd batch file. We need to move this into an MSBuild project and set it as a dependency for all the other projects.

[wasker]

@willduff Should we simply check-in NuGet as part of a solution? This is what Visual Studio's "Enable NuGet package restore" feature does...

[borrillis]

I like the idea of downloading the latest version, due to the aversion of checking in binaries, however, I have worked on other projects that checked in in as well. Checking it in works well, if you need to be strict on version requirements. Forcing users to be connected on their first build isn't a stretch as far as I am concerned.