test(rxSortableColumn): All-column functions

# DON'T MERGE THIS YET

We seem to be experimenting some techmological differences. View the travis logs to see more, as well as the long-form comments in https://github.com/rackerlabs/encore-ui/commit/c298e4fade292e821a5b3958498e3727a43324ab

Useful for when you need all column names, or their sorts by table.

Includes some refactoring for column sorting, which now gets reused.
custom `mapFn` arguments go to `getNamesUsing`, not this.