Fixed high CPU load due to constant rendering in the loop.

This PR fixes issue #25. The discussion of this issue points out that rendering has to occur in the three cases. It seems true, but just calling `renderer.render()` during dragging states by mouse reveals a worse performance than it was initially. It happens when a rendering rate, which depends on number of `mousemove` events, is higher then a possible animation frame rate. The solution is to use `requestAnimationFrame` and invoke it only when there is a need and the previous request is processed ('rendering_requested' attribute of the renderer).
And that has been done.

During resize of the window we can call `renderer.render()` directly for the immediate redraw. It looks better then calling `renderer.requestRender()`.

Below is the CPU load on my PC before this patch:
![before](https://cloud.githubusercontent.com/assets/2686981/5509468/b4156488-87b8-11e4-95dd-52f1b5658df1.png)

and after:
![after](https://cloud.githubusercontent.com/assets/2686981/5509478/caf19a5a-87b8-11e4-80d8-63c2e6e2df09.png)

On the last picture the high CPU load in the end occurs while dragging a state.
`this.rendering_requested = false;` does two things. It defines the attribute of the renderer `rendering_requested` (renderingRequested now) which says if a call for requestAnimFrame was done already. Second, it sets the initial value. This variable could be private if the method `requestRendering` belonged to the object, not to the prototype.