Basic fixes for CalendarServer running on OS X (Mountain) Lion Server

Multiple issues found related to the specifics of the [CalendarServer](http://trac.calendarserver.org) and an [openssl negotiation bug](https://bugs.launchpad.net/ubuntu/+source/openssl/+bug/965371)(?).

So far, to make it work with the subj.:

1. fix requests library digest authentication issue https://github.com/kennethreitz/requests/pull/1242
2. apply [Lukasa's solution](https://github.com/Lukasa/blog-posts/blob/master/posts/Choosing_SSL_Version_In_Requests.md) for the ssl bug workaround: https://github.com/k8n/pycarddav/commit/3cdece219b6a0a77602714ec3759147b6239a87e
3. generalize handling of the 'getcontenttype' prop (CalendarServer gives the "text/vcard; charset=utf-8" string variation: https://github.com/k8n/pycarddav/commit/1d8df6479e57b211f46a4c2193b9353d5af42e8b
4. Fix to initialization issue when working against CalendarServer (propfind infinity error): https://github.com/k8n/pycarddav/commit/2d364766908192ee3bc73fcb4e8b67fc06283883

P.S. an extra commit with a typo fix tagged along (https://github.com/k8n/pycarddav/commit/92b33123342318f5a586ee842762588df9803bb3); sorry.