Added NuGet packaging target in FAKE script.

Would much rather be using the functionality described under "Import MSBuild targets and props files into project" [here](http://docs.nuget.org/docs/creating-packages/creating-and-publishing-a-package).
That way we don't have to add anything into the content folder of the NuGet package just to get the install.ps1 and uninstall.ps1 scripts to run. Don't even need the scripts at all.

But placing the `<UsingTask>` element in a separate `.targets` file changes the path gotten through `BuildEngine.ProjectFileOfTaskNode` to be that `.targets` file. Have yet to find a way to get at the project file that imported the `.targets` file without using a bunch of hacky reflection and reading private members of the TaskHost