Filtered listeners are now triggered on `dc.baseChart.removeFilter` calls

Filtered listener were invoked on `dc.baseChart.addFilter` and
`dc.baseChart.resetFilter` calls but not on `dc.baseChart.removeFilter`
calls. Now filtered listeners are also invoked on
`dc.baseChart.removeFilter` calls.

One caveat:  `_chart.invokeFilteredListener` in `removeFilter` passes
the filter that has been *removed* - this might lead to some strange
behaviour if the listener was expecting to receive only new filters.
Merge pull request #226 from lorcan/master

Filtered listeners are now triggered on `dc.baseChart.removeFilter`