Fixes #25

Now it is possible to use two different calendars:

    In [1]: from ics import Calendar, Event

    In [2]: c = Calendar()

    In [3]: e = Event()

    In [4]: e.name = "test"

    In [5]: e.begin = "2013-11-04T08:15:00+00:00"

    In [6]: c.events.append(e)

    In [7]: c.events
    Out[7]: [<Event 'test' begin:2013-11-04T08:15:00+00:00 end:2013-11-04T08:15:01+00:00>]

    In [8]: d = Calendar()

    In [9]: d.events
    Out[9]: []

    In [10]: c.events
    Out[10]: [<Event 'test' begin:2013-11-04T08:15:00+00:00 end:2013-11-04T08:15:01+00:00>]

    In [11]: c = Calendar()

    In [12]: c.events
    Out[12]: []