WMS getLegendGraphic management in QgsLegend and QgsComposer

Four commits that sequentially add these features

cada6e2 - getLegendGrapic in data provider
71ebde7 - add WMS legend in QgsLegend and add a popup to show it better
a93312f - add configuration option to set WMS legend resolution
ebdf756 - add support in QgsComposer to show WMS legend