Testproject

This pull request includes a test project. 

A basic system test is added for a simple WordPress XML file (`exporttest.wordpress.2013-11-17.xml`).
NUnit is used as test framework, but Approvaltests is also added to make it easier to do automated acceptance/full system tests. 
Does it make sense to you to use NUnit and Approvaltests?
I didn't add a test runner. I believe AppVeyor will run nunit tests automatically. 

I realise that this pull request is quite large. Sorry about that. On the other hand I think it would be really good to have system tests at this point, since testing WordPress xml files manually is quite time consuming. 

I have added Nuget packages directly in the version control. If you prefer it differently let me know.