Use async methods for getting the rrd file

Those commits add async methods to the js library (I'll make a pull request to the original project soon)

The async system can be set on/off by simply modifying CGP.js line 108 (rrdgraph.graph_paint_async(); VS rrdgraph.graph_paint();)

As a result, all rrd are pulled in parallel instead of sequentially.
A side effect of this modification is that "Unresponsive script errors" do not occur any-more when the bandwidth is low.