Switzerland: add rules for canton of Vaud

[brutasse]

Switzerland public holidays are canton-specific:

https://en.wikipedia.org/wiki/Public_holidays_in_Switzerland

Canton of Vaud doesn't have Labor day or boxing day, but has its own
"Lundi du Jeûne Fédéral" in september.

• Tests with a significant number of years to be tested for your calendar.
• Docstrings for the Calendar class and specific methods.
• Calendar country / label added to the README.rst file,
• Changelog amended with a mention like: "Added <country> by @pseudo (#)"

[brutasse]

I chose to expose Vaud in the global workalendar.europe namespace but I'm happy to leave it under workalendar.europe.switzerland if that's more suitable (26 cantons, not sure how many of them have the same rules 😄)

[brunobord]

hey @brutasse, nice to see that you're contributing to this project!

As you may see, I've tackled a few PRs that were on hold for too long, and it resulted in a conflict on the CHANGELOG file. I'm sure you are able to rebase from master and repush your branch ; when it's ready, I'll merge it right away. :o)

[brutasse]

@brunobord my pleasure! Conflicts resolved :)