Add async method for getting the rrd files

[Feandil]

Adding some async methods required to make CGP more responsive (see https://github.com/pommi/CGP/pull/37)

No modification to the existing API, I'm just adding stuff ,) I tried to re-use as much existing code as possible, as you can see in the "splitting" commits.

I'm not good at javascript, thus do not hesitate to make comments

[manuelluis]

Thanks for the patch.

[Feandil]

Thanks :)

I just realised that '"async_paint" is probably not the best choice for a function name. The async part of this function is the fetch. The painting part will not be async, it's still using graph_paint_draw() (sorry)

[manuelluis]

maybe a single function like this?

RrdGraph.prototype.graph_paint = function (async_fetch)
{
        this.graph_paint_init()
        if (async_fetch === undefined || !async_fetch) {
                if (this.data_fetch() === -1)
                        return -1;
                return this.graph_paint_draw()
        } else {
                this.data_fetch_async()
        }
};