Now checks for empty and null values separately

ArgumentNullException is thrown when a null value has been passed;
ArgumentException is thrown if the string is empty.
Refactored test class name

Test class name now matches the name of the class that is being tested.
Added reference and updated test class name
Added unit test project

Includes tests for StringCheck and IdCheck (Validation.cs).
Merge pull request #11 from jorisvervuurt/master

Pull request for unit test project
Merged so everyone can work on implementation
unit test now in project