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.


Btw, the idea of the extension methods is to hide this type of complexity from day-to-day development, so all the code above should be reduced to a method that 
a) creates a tmp folder
b) creates a tmp file inside that folder 

and both of those methods already exists (see other UnitTests or https://github.com/TeamMentor/Master/tree/master/Source_Code/TM_UnitTests for tons of FluentSharp written unit tests)