WMS getLegendGraphic management in QgsLegend and QgsComposer - Developed with funding from Regione Toscana-SITA

Four commits that sequentially add these features

d770843 - getLegendGrapic in data provider
48002db - add WMS legend in QgsLegend and add a popup to show it better
32da13b - add configuration option to set WMS legend resolution
f6a6a0c - add support in QgsComposer to show WMS legend

Developed with funding from Regione Toscana-SITA

previous push discussion on: https://github.com/qgis/QGIS/pull/964