Fixing NUnit detection on 64-bit systems and added user settings

Fixing NUnit detection on 64-bit systems and added user settings to remeber the custom location

On a 64-bit system the detection algorithm doesn't detect NUNit installed in a custom folder. Additionally, if a user has
NUnit installed in a custom location (or in the 64-bit program files folder), they will have to select the NUnit location
with each run.  The new algorithm correctly detects the need to check both program folders by default before asking the
user to choose the location of the NUnit.  Finally the NUnit location is stored as a setting so that the check only needs
to be done once.

I also changed NUnit.Framework from being a direct DLL reference to using the NuGet package.