Serveral suggestions on API of SlidingCategoryDataset

[yfwz100]

I've just used JFreeChart and found it really useful. Thanks for the authors/contributors.

I have serval suggestions on SlidingCategoryDataset:

• Use Java Generic Type to indicate the type of underlying dataset.
  Let others know about the true type of the underlying dataset and provide information to operate on the underlying dataset.
• Underlying dataset's change also reflect to the changes of SlidingCategoryDataset.
  This enable developers to use SlidingCategoryDataset as a wrapper instead of a actual dataset type.
• Add options to track the latest serveral records or first serveral records.
  This is useful when trying to plot the changes in the data stream.

I don't know whether there's already any alternative implementation to the problems above in the library but I think there should be some notice on the implementation of SlidingCategoryDataset. :)