International day feature

[IriannaPapoutsi]

This PR introduces the brand new international day feature mentioned on #869 issue. More specifically:

1. It gives the user the chance to learn the International Day either for today's date, by instantly getting the current date from the pc, or for an option date of his own.
2. It includes the proper documentation.
3. It follows PEP8 standards.
4. It adds a test class that validates the functionality of the source code.

Below you can see how the feature actually works...