New function in sigma.utils: zoomTo. Also a fix in the renderers.

I needed to change the animate param that you commented to a dictionary, you will see it in the code, I hope you like it.
A little thing, to obtain the settings for the `zoomMin` and the `zoomMax`  it will be ok if it is used the settings of the camera instead of the sigma instance?

And about of the fix of the renderers, the thing is that in some environments, like Travis, when we used (in our lab's projects) GUI testing using Firefox, this line `canvas.getContext('webgl')` returns `null`, so by lazy evalutaion goes to the next one, `canvas.getContext('experimental-webgl')`, and that one crash with this error: `Component returned failure code: 0x80004005 (NS_ERROR_FAILURE) [nsIDOMHTMLCanvasElement.getContext]`. So for test the use of Sigma that we do, we need to capture that error.