Added support for option `labelsDateUTC` to the date tick picker.

    * dygraph-tickers.js:
      simplified date tick generation. `Dygraph.getDateAxis`
      now produces nice ticks both in UTC and local time.

    * dygraph-utils.js:
      removed `Dygraph.dateSetters` and `Dygraph.setDateSameTZ`
      because they are not needed any more.
Added UTC date label tests and adapted existing ones.

    * tests/labelsDateUTC.html:
      New test/demo for the option  `labelsDateUTC`.
      Display several days of data according to UTC and to local time,
      and compare the tick placement and the label format.

    * tests/x-axis-formatter.html:
      Added `opts` argument to `axisLabelFormatter` function
      (used by inner `Dygraphs.dateAxisLabelFormatter` call).