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.
Just to be sure:
The plot in the [home page][home] page and in the [range selector demo][demo]
show the labels as `MMM YY` (e.g. `Aug 08`).
With your comment we will change that format to `MMM YYYY` (e.g `Aug 2008`).
Shall we do it?

  [home]: http://dygraphs.com/
  [demo]: http://dygraphs.com/gallery/#g/range-selector