Allow ConfusionMatrix to be Writable (via MatrixWritable)

ConfusionMatrix does not support Writable. This patch adds that feature. ConfusionMatrix is a subclass of MatrixWritable.

Since ConfusionMatrix is somewhat less than useful without the row/column labels, and MatrixWritable does not support writing bindings (it only saves numbers), this patch fixes both.

Includes unit test for ConfusionMatrix (previously missing) which includes exercise of MatrixWritable support for numbers and labels. (There is no independent unit test for MatrixWritable.)