Bug 974554 - CalDAV parser should accept ical inside xml comments nested  in <calendar-data> elements r=millermedeiros

Yahoo! returns `<calendar-data>` elements that look like

```
...
<calendar-data><![CDATA[BEGIN:VCALENDAR
...
]]></calendar-data>
...
```

and right now we choke there since we're ignoring comments and expecting for things to look more like
```
...
<calendar-data>
BEGIN:VCALENDAR
...
</calendar-data>
...
```

But both are valid according to http://tools.ietf.org/html/rfc4791 9.6