Hi @gaye,

Do the factory, we will have these profit.
1. We could have many implementations of CalDAV at same time in the future.
2. We could switch the different CalDAV server at testing rumtime.
3. If we would like to have the above profit, do the factory is easier to achieve it.

So if we make sure that we don't need to use one more implementations of CalDAV server at same time,
we could just do `new Radicale()`.

How does @gaye think?