Dygraph.dateString_: shows milliseconds if any. (#774)

* [utils] dateString_ display Milliseconds if any

* [utils] fix dateAxisLabelFormatter milliseconds

* [util] dateString_: compute ms in frac

* [utils] clean hmsString_ format

* [utils] add milliseconds padding in hmsString_

* [develop] add note on npm install

* [tests] add labelsDateMilliseconds test

* [utils] add millis in dateAxisLabelFormatter

* [tests] fix requested changes in labelsDateMillis

* [auto_tests] add date_formats testMillisecondsDate