Added support for UTC date time labels.

    * dygraph-utils.js:
	new utc switch parameter for `Dygraph.hmsString_`
        and `Dygraph.dateString_`.

    * dygraph.js:
        new utc date axis formater `Dygraph.dateUTCAxisFormatter`.
Merge branch 'utc_datetime_labels-issue_498'