Add xUnit test project

[sayedihashimi]

We should add a xUnit Test generator.

The reason why we are using xUnit instead of MSTest here is because xUnit is very popular on xplat and is gaining ground in Windows as well IMO.

Here is a link which describes how to create a new xUnit project from a class library project http://xunit.github.io/docs/getting-started.html.

Ideally each PR will have mocha test cases that cover the additions. For examples on how to implement this see the tests in generator-aspnet https://github.com/OmniSharp/generator-aspnet/tree/master/test.