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.