UTC datetime ticks and labels (fix issue 498)

Add the feature of placing and displaying time labels according to UTC instead of local time.
Provide a new option `labelsDateUTC`, and adapt the label and value date formatters, 
and the date tick computation accordingly:

 * The option `labelsDateUTC` is coherent whith other options like `labelsKMG`.

 * The date tick computation is simpler and homogeneous, and always 
   places the ticks at nice instants.

 * When using local time, ticks are properly generated on DST transitions
   (with some minor differences with regard to the original implementation).

This is a better implementation of some ideas in pull request #291, where
only the labels are formatted according to UTC. In this version, the ticks are 
also placed according to UTC at nice calendar positions.
may as well pull these into an object outside of `Dygraph.getDateAxis`, so that they're not redefined on every call to this function. I'm thinking:

```javascript
Dygraph.DateAccessors_ = {
  utc: {
    weekday: …,
    decompose_date: …,
    compose_date: …
  },
  local:  {
    weekday: …,
    decompose_date: …,
    compose_date: …
  }
};
```

and then:

```javascript
var o = Dygraph.DateAccessors_[utc ? 'utc' : 'local'];
var weekday = o.weekday, decompose_date = o.decompose_date, compose_date = o.compose_date;
```

so that this method is more focused on axis generation than date composition/decomposition. This would also facilitate unit testing of the accessors.