#315 Permit sorted datasets and allow (multi)pie plots to be subclassed

[Scott Dudley]

This patch makes it easier for callers to subclass the pie and multipie plots outside of the JFreeChart core, allows for DateAxis to be more easily subclassed, and allows the caller to supply custom comparators for various categories and other elements in datasets.

• ChartFactory#createMultiplePieChart permits the caller to supply its own MultiplePiePlot (useful when client wants to subclass the plot)

• MultiplePiePlot breaks out a number of calculation-based methods (related to the rows and columns of pies) as protected methods to allow subclasses of the plot.

• DateAxis#calculateDateForPosition and DateAxis#correctTickDateForPosition are promoted from private to protected to make it easier for clients to work with subclasses date axis

• KeyedValues2D permits the caller to supply custom row and column comparators in order to support the addition of categories rendered in a specific order.

• DefaultCategoryDataset permits the caller to supply a custom row comparator

• TimeTableXYDataset permits the caller to supply a custom column key comparator