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.


See this coffee script unit test I wrote recently that is doing a similar check https://github.com/TeamMentor/TM_4_0_Design/blob/master/build/tests/test-IO-Actions.coffee