Caldav Support WIP

Hi Mike, 

We needed calDAV support in node.js for cozy.io, so I went ahead and got started on it. 

It still needs a lot of love, what I see missing : 
- Tests, tests, tests
- Freebusy and Calendar queries
- A lot of "CardDAV", "card", that I miss in comments

However, we probably won't work on it for a little while as It seems to work for our basic usage.

So this is not really a pull request per-se, but a way to fire up the discussion and to get you or may be someone else involved in this branch.

Thanks,
Romain