Remove and re-add the NSass project, update MVC Tests project to be VS20...

...13-compatible.

The aim of this PR is to try to fix the broken build, which has the following error:
```
System.IO.FileNotFoundException : Could not load file or assembly 'NSass.Wrapper.x64.dll' or one of its dependencies. The specified module could not be found.
   at NSass.SassCompiler..ctor()
```

I couldn't reproduce the error on my machine by deleting the NSassWrapper folder from the output directory.  When I did, I got this error, which is pretty different:
```
The type initializer for 'NSass.SassCompiler' threw an exception.
  ----> System.InvalidOperationException : Found NSass.Wrapper.proxy.dll which cannot exist. Must instead have NSass.Wrapper.x86.dll and NSass.Wrapper.x64.dll. Check your build settings.C:\Users\jkodroff.INTERNAL.000\Code\SquishIt\SquishIt.Tests\bin\Debug\NSass.Wrapper
	at NSass.SassCompiler..ctor()
```

It may be totally unrelated, but I noticed that the tests would not build in Release mode, so I removed and re-added the reference and they did build in release mode.

In opening the solution, I also automatically updated the MVC project so that it's VS2013-compatible.  (Couldn't be helped - I only have 2013 installed.)