Implement xCal and xCard support (bis)

Useful links:
  * [ ] [RFC6321](https://tools.ietf.org/html/rfc6321), xCal: The XML Format for iCalendar,
  * [ ] [RFC6351](https://tools.ietf.org/html/rfc6351), xCard: vCard XML Representation.

:exclamation: **Not ready to merge**, this is a WIP :exclamation:

To-do:
  * [x] require sabre/xml,
    * [x] update `composer.json`,
    * [x] sabre/xml requires PHP5.4, hence update sabre/vobject (done by Composer?),
      * [x] if requires PHP5.4, we can use the [`JsonSerializable`](http://php.net/jsonserializable) everywhere (all top-classes having a `jsonSerialize` method),
      * [x] update code accordingly (e.g. short array syntax),
      * [ ] update documentation accordingly,
  * [x] create the helper in `Sabre\VObject\Reader`,
  * [ ] add XML serializer helpers everywhere (e.g. in children of `Sabre\VObject\Node`),
  * [ ] timezones used by xCal and xCard are differents, must check the compatibility,
  * [ ] create the reader (parser):
    * [ ] support properties:
        * [x] support property name,
        * [x] support property value,
        * [x] support property type,
        * [ ] support several values,
    * [x] support components.

Code review:
  * usage of iterators has been heighten,
  * clean code,
  * better performance when possible.


Left apart for now:
  * [ ] [RFC6321, section 3.1. Pre-Processing](https://tools.ietf.org/html/rfc6321#section-3.1),
  * [ ] `LIBXML_*` options when reading or writing.
why use `null` as default, when it is not a valid value?