Implement zoom and scroll on plot

[Juan Pablo Caram]

zoom_fit: Fit the plot within axes with 5% margins. Hide scroll bars.

zoom(scale, center): axes_width = total_width/scale, range = [center - axes_width/2, center + axes_width/2]. Adjust scroll bars appropriately.

[Juan Pablo Caram]

Use of mouse-wheel is buggy in GTK+ 3: See

https://github.com/matplotlib/matplotlib/issues/2668

http://askubuntu.com/questions/356640/ubuntu-13-10-mouse-scroll-wheel-does-in-work-in-gtk3-apps

[Juan Pablo Caram]

Implemented feature with toolbar and keys (1,2 and 3). Toolbar zooms and maintains current center. Keys maintain the point under the cursor at the same location. Scrolling can be done by zooming-out and zooming-in again in the desired location.