Add ability to configure time axis tick labels.

[matoos32]

• Implements enhancement #61
• Adds ChartBuilder ability to set any desired java.text.DateFormat.
• Allows using the Java standard SimpleDateFormat and other DateFormat derived classes in the JDK.
• Extracts previously nested gapless chart logic for custom minimal date format to new MinimalDateFormat class.
• MinimalDateFormat now usable in both gapless and with-gap time series charts.
• Adds ChartBuilder ability to toggle the ValueAxis.setVerticalTickLabels(boolean) on the time axis.
• Updates JFreeChartBuilderDemo demonstration to emphasize locales.

[matoos32]

Tested on Windows 10 with JDK 11 by visually inspecting the demo app. I didn't observe any problems with these changes.

[matoos32]

All review findings addressed. Merging.