Added a singledispatch as a dependency on older python installs.

Needed by 'Paint data' widget.
Support unknows in DiscreteVariable.to_val.
Covert sql values to correct str representation.
Replace references to classVar with class_var.
Construct the same type of table in OWMPR widget.
Share code between sql an nupy computation.

Fixes display of class colors on sql data.
Removed no class_var warning.

Widget works just fine with data that does not have class_var. (It just
won't show colors).
Do not crash if pearson coloring is selected.
Disable statistics.
Compute min and span for discrete variables.
Do not draw lines with missing values.
Treat not discretized discrete variables as ordinal.
Skip lines with unknown values when computing contingencies.
Fix for SQL data in Mosaic.
Fix for missing values.
Skip packing settings that are not present on instance.