Added updated Nuget package spec and Nuget readme file

The AmplifyJS Nuget package has been needing to be updated with the latest code for a while. This request adds the `nuspec` file into Source control (as well as the associated readme) and maps the built output of Amplify to match the existing Nuget structure. The only new addition is the `amplify` folder with individual components inside. The finished package files look like this:

```
/content
  /Scripts
    /amplify
       amplify.core.js
       amplify.core.min.js
       amplify.request.js
       amplify.request.min.js
       amplify.store.js
       amplify.store.min.js
    amplify-vsdoc.js
    amplify.js
    amplify.min.js
    GPL-LICENSE.txt
    MIT-LICENSE.txt
    README.txt
```

After discussing with @elijahmanor I opted to not put the `amplify.js` and `amplify.min.js` files in the `/amplify` directory so those upgrading from an older version of the package wouldn't have trouble with the files moving.

If @jcreamer898 or anyone else with more experience in .NET and Nuget could chime in on this commit, it would be appreciated!