integrate Google and Bing background layers

Hi,

together with Lars Lingner we started to add support for Google and Bing as background layers. Both is working in general (see http://5.9.122.75/qgiswebclient.html?map=/home/dassau/schulung/deutschland.qgs), if you use the new GlobalOptions.js.templ-3857, set var enableCommercialMaps = true and add valid BingApiKey to WebgisInit.js.

At this stage we would like to add it to qgis-web-client repo for further testing and improving, because we currently focus 2 problems, where others, more familiar with the qgis-web-client might have ideas:

* Transparency is not working properly with Layers from QGIS server
* Background Layers (Google and Bing) are selectable as checkbox but this should be radio buttons

Furthermore I think if would be nice to add other layers, such as OSM to the list of supported background layers.

A note: To make it work properly we also had to change the currently used/smaller Openlayers.js library with the original bigger one.