Document level clock definition...

This is probably a little rough around the edges, but is a first cut of a `clock` property for specifying the default clock settings for a czml document or stream.  The general idea is that we add `document` as a reserved `id` name and `document` will have unique properties that define "layer-wide" data. (clock being the only one so far).

The only special thing about clock is that none of it's values are time-varying; simply because I don't think it makes any sense.  How could start time vary over time?  The end result looks like this.  When encountered, the client would switch to the new values immediately.

```
{
  "id":"document",
  "clock":{
    "interval":"2013-03-20T21:58:58Z/2013-03-21T21:58:58Z",
    "currentTime":"2013-03-21T09:58:58Z",
    "step":"SYSTEM_CLOCK_MULTIPLIER",
    "multiplier":60.0,
    "range":"LOOP_STOP"
  }
}
```

I know both me and @emackey need this in our respective projects.  Note, this is not related to stored views and tours, which will happen at some point in the future; but I would hope to reuse this clock schema there as well.

CC @kring @shunter