Added description and unit tests to IO_ExtensionMethods_Folders.cs ( https://github.com/o2platform/FluentSharp/issues/10)

I tried to add also extra checks for some functions, hope it is ok . There is also a fix for a problem in String_ExtensionMethods.cs which I missed in the previous pull request.


you can replace all this with the ```temporaryFolderPath.delete_Folder_Recursively()``` (since it is that extension method that should check if temporaryFolderPath is null or doesn't exist)

also can you put this method close to the ```TestFixtureSetup``` since they are related