Various improvements, unit testing etc.

[abjerner]

As we talked about quite some time ago, I have now introduced helper classes for YouTube and the date methods. The extension methods now call the helper methods so that the actual logic isn't implemented twice.

DateHelpers currently doesn't have all the methods yet, but it is a step in the right direction. DateHelpers and DateExtensionMethods now also has some culture dependent methods like getting the name of the day for a specific culture.