XUnitConverter: Add support for converting TestInitialize and TestCleanup attributes to xUnit equivalent #247

I used the converter to successfully convert our MSTest projects to xUnit, except that I noticed that TestInitialize and TestCleanup attributes were not converted, so I've added that in my own branch (here).

I think it would be a good addition to the converter, are you guys willing to merge that in? I think the branch requires some work to get it up to PR level, but I don't want to do that if it's not going to be merged ;)