Require php calendar extension to prevent breaking sites

[Jarlskov]

I've accidentally taken down a production site because the server did not have the calendar extension installed, and I was not aware that I needed it (or that it existed actually).
Since it's a hard dependency for the library to function, that should be more obvious :-)

[stelgenhof]

@Jarlskov Sorry to hear that your site was taken down because of this :( You are correct that it is not clear that the calendar extension is actually required. If I recall, I wanted to stay away from such dependency but think I ended up using it anyway.

Your PR is good in that it will enforce the use of the calendar extension. Now that I am thinking of it, I think I want to provide an implementation of the easter_days function within Yasumi that would replace the need for the extension after all.

[Jarlskov]

No worries, that's our own fault. We were able to roll back quickly, and it showed some flaws in our test procedures :-)

I'm not sure why you would want to maintain existing functionality, but that's up to you how you want to solve it, but I think it would make sense to add something like this until you get around to implementing the easter_days functions. I don't believe this PR should cause any BC breaks.

[stelgenhof]

Ah yes, I see. Merging this PR will indeed not cause any BC issues. Implementing a easter_days function as part of Yasumi isn't that difficult and planning on releasing it pretty soon :)

[stelgenhof]

Fallback easter_days calculation implemented in case the PHP calendar extension is not loaded. (See PR #55)