do not filter for brush events with zero brush length

Currently, if you click (without dragging) on the rangeChart associated with a coordinate grid chart, the rangeChart loses its brush. This is because brushIsEmpty returns true for a brush with an equal start and end.

To replicate this, load the example page (http://nickqizhu.github.io/dc.js/), filter the line graph using the rangeChart, and then click anywhere on the rangeChart. The brush will disappear. If this behavior is intentional, feel free to ignore this pull request.

Also, brushing is now _chart._brushing. This was necessary for testing.